Update pip installation step in docs to use https instead of unencrypted git

[RobertKielty]

On https://powerline.readthedocs.io/en/master/installation.html#pip-installation

I attempted to install powerline-status by running

pip install --user git+git://github.com/powerline/powerline

and it generated the following error:

 pip install --user git+git://github.com/powerline/powerline
Collecting git+git://github.com/powerline/powerline
  Cloning git://github.com/powerline/powerline to /tmp/pip-req-build-8lfdb5aw
  Running command git clone --filter=blob:none --quiet git://github.com/powerline/powerline /tmp/pip-req-build-8lfdb5aw
  fatal: remote error:
    The unauthenticated git protocol on port 9418 is no longer supported.
  Please see https://github.blog/2021-09-01-improving-git-protocol-security-github/ for more information.
  error: subprocess-exited-with-error
  
  × git clone --filter=blob:none --quiet git://github.com/powerline/powerline /tmp/pip-req-build-8lfdb5aw did not run successfully.
  │ exit code: 128
  ╰─> See above for output.
  
  note: This error originates from a subprocess, and is likely not a problem with pip.
error: subprocess-exited-with-error

× git clone --filter=blob:none --quiet git://github.com/powerline/powerline /tmp/pip-req-build-8lfdb5aw did not run successfully.
│ exit code: 128
╰─> See above for output.

For background see the referenced github blog post in the error message

https://github.blog/2021-09-01-improving-git-protocol-security-github/

Changing the installation step to use git+https instead git+git seems to work

pip install --user git+https://github.com/powerline/powerline

[RobertKielty]

I was going to submit a PR for this but I now see that there is already a PR merged onto master to cover off this issue.

[RobertKielty]

So it's now just remains to update the following ?

https://powerline.readthedocs.io/en/master/installation.html#pip-installation

[PH111P]

Yes indeed that should be fixed.

However, the build script for the docs is somehow broken (on readthedocs) and I have now means to fix it. In particular, the docs at powerline.readthedocs.io are awfully out-of-date in some places, sorry for that.