Fix pip reinstallation on windows due to constraints (#6756)

[patchback]

This is a backport of PR #6756 as merged into master (72d3d4b).

What do these changes do?

Fix for constantly failing CI on windows agents.

Description

We have pinned pip version in constraints.txt file:

aiohttp/requirements/constraints.txt

Line 260 in 08ca779

pip==21.3.1

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]

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

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