pip install fails "Could not find a version that satisfies the requirement"

Question

I'm trying to install some packages using pip, but keep getting the error bellow.

 pip3 install flask
Collecting flask
  Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProtocolError('Connection aborted.', ConnectionResetError(104, 'Connection reset by peer'))': /simple/flask/
  Retrying (Retry(total=3, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProtocolError('Connection aborted.', ConnectionResetError(104, 'Connection reset by peer'))': /simple/flask/
  Retrying (Retry(total=2, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProtocolError('Connection aborted.', ConnectionResetError(104, 'Connection reset by peer'))': /simple/flask/
  Retrying (Retry(total=1, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProtocolError('Connection aborted.', ConnectionResetError(104, 'Connection reset by peer'))': /simple/flask/
  Retrying (Retry(total=0, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProtocolError('Connection aborted.', ConnectionResetError(104, 'Connection reset by peer'))': /simple/flask/
  Could not find a version that satisfies the requirement flask (from versions: )
No matching distribution found for flask

I've tried to install several packages (numpy, scipy and matplotlib), all of them failed with a similar error message.

I've tried to use include --trusted-host pypi.org --trusted-host files.pythonhosted.org in the command (as in this answer) and using easy_install, none of it worked.

It's not an issue with my firewall, as it still doesn't work after having turned it off.

P.S.: I'm using a virtual environment, but, that does not seem to influence the outcome. And, when I try to access pypi, through the browser, most of the time it says that the website can't be reached.