[PR #6756/72d3d4b1 backport][3.9] Fix pip reinstallation on windows due to constraints

[patchback[bot]]

This is a backport of PR #6756 as merged into master (72d3d4b1f68cca5ad15ef50bffb0419b798c7f23).

What do these changes do?

Fix for constantly failing CI on windows agents.

Description

We have pinned pip version in constraints.txt file: https://github.com/aio-libs/aiohttp/blob/08ca779e9abdb6d558cdd1647fdca17cfe979711/requirements/constraints.txt#L260

So when you call pip install ... -c constraints.txt it will downgrade/upgrade pip depending on the currently installed version. This works fine on all platforms except windows, which forces you to explicitly use python -m pip install ... when dealing with the pip package itself.

Since python -m pip install ... command is a completely legit replacement for pip install ... (examples from the pip wiki), I decided to update Makefile with it.

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (3.9@d4814a9). Click here to learn what that means. The diff coverage is n/a.

:exclamation: Current head 72d3d4b differs from pull request most recent head ddd4405. Consider uploading reports for the commit ddd4405 to get more accurate results

@@          Coverage Diff           @@
##             3.9    #6759   +/-   ##
======================================
  Coverage       ?   93.44%           
======================================
  Files          ?      104           
  Lines          ?    30627           
  Branches       ?     3076           
======================================
  Hits           ?    28620           
  Misses         ?     1838           
  Partials       ?      169           

Flag	Coverage Δ
unit	93.36% <0.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update d4814a9...ddd4405. Read the comment docs.