For those who are on windows and were struggling to find an working answer here's mine: pynput.

Here is the pynput official "Monitoring the keyboard" source code example:

from pynput.keyboard import Key, Listener

def on_press(key):
    print('{0} pressed'.format(
        key))

def on_release(key):
    print('{0} release'.format(
        key))
    if key == Key.esc:
        # Stop listener
        return False

# Collect events until released
with Listener(
        on_press=on_press,
        on_release=on_release) as listener:
    listener.join()

The function above will print whichever key you are pressing plus start an action as you release the 'esc' key. The keyboard documentation is here for a more variated usage.

Markus von Broady highlighted a potential issue that is: This answer doesn't require you being in the current window to this script be activated, a solution to windows would be:

from win32gui import GetWindowText, GetForegroundWindow
current_window = (GetWindowText(GetForegroundWindow()))
desired_window_name = "Stopwatch" #Whatever the name of your window should be

#Infinite loops are dangerous.
while True: #Don't rely on this line of code too much and make sure to adapt this to your project.
    if current_window == desired_window_name:

        with Listener(
            on_press=on_press,
            on_release=on_release) as listener:
            listener.join()

You could also use PyAutoGui to send a virtual key presses.

Here's the documentation: https://pyautogui.readthedocs.org/en/latest/

import pyautogui


pyautogui.press('Any key combination')

You can also send keys like the shift key or enter key with:

import pyautogui

pyautogui.press('shift')

Pyautogui can also send straight text like so:

import pyautogui

pyautogui.typewrite('any text you want to type')

As for pressing the "A" key 1000 times, it would look something like this:

import pyautogui

for i in range(999):
    pyautogui.press("a")

alt-tab or other tasks that require more than one key to be pressed at the same time:

import pyautogui

# Holds down the alt key
pyautogui.keyDown("alt")

# Presses the tab key once
pyautogui.press("tab")

# Lets go of the alt key
pyautogui.keyUp("alt")