Note: This solution is a complete hack. And works only for requests < 2.28 (https://github.com/psf/requests/pull/6074)

Short answer: Set the CURL_CA_BUNDLE environment variable to an empty string.

Before:

$ python
import requests
requests.get('http://www.google.com')
<Response [200]>

requests.get('https://www.google.com')
...
File "/usr/local/lib/python2.7/site-packages/requests-2.17.3-py2.7.egg/requests/adapters.py", line 514, in send
    raise SSLError(e, request=request)
requests.exceptions.SSLError: ("bad handshake: Error([('SSL routines', 'SSL3_GET_SERVER_CERTIFICATE', 'certificate verify failed')],)",)

After:

$ CURL_CA_BUNDLE="" python
import requests
requests.get('http://www.google.com')
<Response [200]>

requests.get('https://www.google.com')
/usr/local/lib/python2.7/site-packages/urllib3-1.21.1-py2.7.egg/urllib3/connectionpool.py:852: InsecureRequestWarning: Unverified HTTPS request is being made. Adding certificate verification is strongly advised. See: https://urllib3.readthedocs.io/en/latest/advanced-usage.html#ssl-warnings InsecureRequestWarning)
<Response [200]>

How it works

This solution works because Python requests overwrites the default value for verify from the environment variables CURL_CA_BUNDLE and REQUESTS_CA_BUNDLE, as can be seen here:

if verify is True or verify is None:
verify = (os.environ.get('REQUESTS_CA_BUNDLE') or
    os.environ.get('CURL_CA_BUNDLE'))

The environment variables are meant to specify the path to the certificate file or CA_BUNDLE and are copied into verify. However, by setting CURL_CA_BUNDLE to an empty string, the empty string is copied into verify and in Python, an empty string evaluates to False.

Note that this hack only works with the CURL_CA_BUNDLE environment variable - it does not work with the REQUESTS_CA_BUNDLE. This is because verify is set with the following statement:

verify = (os.environ.get('REQUESTS_CA_BUNDLE') or os.environ.get('CURL_CA_BUNDLE'))

It only works with CURL_CA_BUNDLE because '' or None is not the same as None or '', as can be seen below:

print repr(None or "")
# Prints: ''
print repr("" or None )
# Prints: None

I have the exact same issue on my network. I did this to install pillow:

pip install Pillow --trusted-host pypi.python.org --index-url=http://pypi.python.org/simple/

...and it worked great for me. Hope it helps.

From the documentation:

requests can also ignore verifying the SSL certificate if you set verify to False.

>>> requests.get('https://kennethreitz.com', verify=False)
<Response [200]>

If you're using a third-party module and want to disable the checks, here's a context manager that monkey patches requests and changes it so that verify=False is the default and suppresses the warning.

import warnings
import contextlib

import requests
from urllib3.exceptions import InsecureRequestWarning

old_merge_environment_settings = requests.Session.merge_environment_settings

@contextlib.contextmanager
def no_ssl_verification():
    opened_adapters = set()

    def merge_environment_settings(self, url, proxies, stream, verify, cert):
        # Verification happens only once per connection so we need to close
        # all the opened adapters once we're done. Otherwise, the effects of
        # verify=False persist beyond the end of this context manager.
        opened_adapters.add(self.get_adapter(url))

        settings = old_merge_environment_settings(self, url, proxies, stream, verify, cert)
        settings['verify'] = False

        return settings

    requests.Session.merge_environment_settings = merge_environment_settings

    try:
        with warnings.catch_warnings():
            warnings.simplefilter('ignore', InsecureRequestWarning)
            yield
    finally:
        requests.Session.merge_environment_settings = old_merge_environment_settings

        for adapter in opened_adapters:
            try:
                adapter.close()
            except:
                pass

Here's how you use it:

with no_ssl_verification():
    requests.get('https://wrong.host.badssl.example/')
    print('It works')

    requests.get('https://wrong.host.badssl.example/', verify=True)
    print('Even if you try to force it to')

requests.get('https://wrong.host.badssl.example/', verify=False)
print('It resets back')

session = requests.Session()
session.verify = True

with no_ssl_verification():
    session.get('https://wrong.host.badssl.example/', verify=True)
    print('Works even here')

try:
    requests.get('https://wrong.host.badssl.example/')
except requests.exceptions.SSLError:
    print('It breaks')

try:
    session.get('https://wrong.host.badssl.example/')
except requests.exceptions.SSLError:
    print('It breaks here again')

Note that this code closes all open adapters that handled a patched request once you leave the context manager. This is because requests maintains a per-session connection pool and certificate validation happens only once per connection so unexpected things like this will happen:

>>> import requests
>>> session = requests.Session()
>>> session.get('https://wrong.host.badssl.example/', verify=False)
/usr/local/lib/python3.7/site-packages/urllib3/connectionpool.py:857: InsecureRequestWarning: Unverified HTTPS request is being made. Adding certificate verification is strongly advised. See: https://urllib3.readthedocs.io/en/latest/advanced-usage.html#ssl-warnings
  InsecureRequestWarning)
<Response [200]>
>>> session.get('https://wrong.host.badssl.example/', verify=True)
/usr/local/lib/python3.7/site-packages/urllib3/connectionpool.py:857: InsecureRequestWarning: Unverified HTTPS request is being made. Adding certificate verification is strongly advised. See: https://urllib3.readthedocs.io/en/latest/advanced-usage.html#ssl-warnings
  InsecureRequestWarning)
<Response [200]>