Add tests for rez_pip and drop support for installing Python 2 packages

[JeanChristopheMorinPerso]

Add tests for rez_pip and drop support for installing Python 2 packages.

We had to drop support for installing Python 2 packages because when running pip with --python-version and all the other flags (--platform, etc), pip doesn't evaluate the markers on the dependencies based on the information provided. It uses the current interpreter's values... In other words, if we run <sys.executable> pip.pyz install <package> --python-version 2.7 where sys.executable is a Python 3 executable, we will get the dependencies for Python 3 and not 2! See pypa/pip#11664 for more details.

We could potentially hack a solution together, for example by creating a sitecustomize or usercustomize file, or maybe even a .pth file that would override the sys and os values used by markers... But that would be quite fragile.

Lastly, the version of pip we need isn't compatible with Python 2.

This means that dropping support for installing Python 2 packages is our only option right now.

[codecov]

Codecov Report

Merging #39 (30b4566) into main (093ae3d) will increase coverage by 2.75%.
The diff coverage is 100.00%.

❗ Current head 30b4566 differs from pull request most recent head d8b16ff. Consider uploading reports for the commit d8b16ff to get more accurate results

@@            Coverage Diff             @@
##             main      #39      +/-   ##
==========================================
+ Coverage   70.87%   73.63%   +2.75%     
==========================================
  Files           8        8              
  Lines         618      622       +4     
  Branches      123      124       +1     
==========================================
+ Hits          438      458      +20     
+ Misses        170      154      -16     
  Partials       10       10              

Impacted Files	Coverage Δ
src/rez_pip/pip.py	100.00% <100.00%> (+34.37%)	⬆️

... and 2 files with indirect coverage changes