Deprecate call to `setup.py install` when building a wheel failed for source distributions without pyproject.toml

[sbidoul]

This is one of a series of deprecations meant to collect feedback on the ultimate goal of always installing source distributions by first building a wheel and then installing from it.

This specific issue is about the case when pip calls setup.py bdist_wheel and that fails.
In that case, pip currently displays the build error and continues installation by attempting a setup.py install.

In a future version, pip will not attempt setup.py install in that case, and fail the installation right away.

Towards #8102

[djhenderson]

py -3.9 -m pip install -U pillow

fails, includes these messages:

  ERROR: Failed building wheel for pillow
  Running setup.py clean for pillow
Failed to build pillow
DEPRECATION: Could not build wheels for pillow which do not use PEP 517. pip will fall back to legacy 'setup.py install' for these. pip 21.0 will remove support for this functionality. A possible replacement is to fix the wheel build issue reported above. You can find discussion regarding this at https://github.com/pypa/pip/issues/8368.
Installing collected packages: pillow
    Running setup.py install for pillow ... error
    ERROR: Command errored out with exit status 1:

using:

> py -3.9 -m pip --version
pip 20.2 from C:\Users\Doug\AppData\Local\Programs\Python\Python39\lib\site-packages\pip (python 3.9)

> py -3.9
Python 3.9.0b5 (tags/v3.9.0b5:8ad7d50, Jul 20 2020, 18:35:09) [MSC v.1924 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>>

On windows 10 pro 1909, full patched as of 2020-07-30.

This has been reported as python-pillow/Pillow#4827

[uranusjr]

@djhenderson Your issue is not related. The setup.py install fallback deprecation simply shows a warning, and would not affect the error you see.

[pradyunsg]

Thanks for clearly providing all the relevant information here @djhenderson. I think the issue tracker for Pillow (where you've filed an issue) is the correct place to get help with your issue. :)