On macOS, use subprocess.run to pipe your text to pbcopy:

import subprocess 
data = "hello world"
subprocess.run("pbcopy", text=True, input=data)

It will copy "hello world" to the clipboard.

I didn't have a solution, just a workaround.

Windows Vista onwards has an inbuilt command called clip that takes the output of a command from command line and puts it into the clipboard. For example, ipconfig | clip.

So I made a function with the os module which takes a string and adds it to the clipboard using the inbuilt Windows solution.

import os
def addToClipBoard(text):
    command = 'echo ' + text.strip() + '| clip'
    os.system(command)

# Example
addToClipBoard('penny lane')

# Penny Lane is now in your ears, eyes, and clipboard.

As previously noted in the comments however, one downside to this approach is that the echo command automatically adds a newline to the end of your text. To avoid this you can use a modified version of the command:

def addToClipBoard(text):
    command = 'echo | set /p nul=' + text.strip() + '| clip'
    os.system(command)

If you are using Windows XP it will work just following the steps in Copy and paste from Windows XP Pro's command prompt straight to the Clipboard.