Rely on venv instead of virtualenv

Once we drop support for Python 2 we could start relying on venv instead of virtualenv. However, there are still a few hiccups with using venv:

• on Ubuntu 18.04 "python3 -m venv mongovenv" fails with this error:

  [2020/11/20 21:34:33.329] Finished 'shell.exec' in "run aws auth test with regular aws credentials" in 13.76142158s

  [2020/11/20 21:34:33.329] Running command 'shell.exec' in "run aws auth test with assume role credentials" (step 4.1 of 8)

  [2020/11/20 21:34:33.408] The virtual environment was not created successfully because ensurepip is not

  [2020/11/20 21:34:33.408] available. On Debian/Ubuntu systems, you need to install the python3-venv

  [2020/11/20 21:34:33.408] package using the following command.

  [2020/11/20 21:34:33.408] apt-get install python3-venv

  [2020/11/20 21:34:33.408] You may need to use sudo with that command. After installing the python3-venv

  [2020/11/20 21:34:33.408] package, recreate your virtual environment.

  [2020/11/20 21:34:33.408] Failing command: ['/data/mci/751269f52662093748413aa179689d98/src/mongovenv/bin/python3', '-Im', 'ensurepip', '--upgrade', '--default-pip']

• On Windows "C:/python/Python38/python.exe -m venv mongovenv" succeeds but attempting to activate the venv fails with this error:

  [2020/11/20 21:34:02.310] Running command 'shell.exec' in "run aws auth test with assume role credentials" (step 4.1 of 8)

  [2020/11/20 21:34:06.700] mongovenv/Scripts/activate: line 3: $'\r': command not found