[WIP] bpo-1294959: setup.py uses sys.platlibdir

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setup.py now uses sys.platlibdir to configure the compiler library
directories and to locate the termcap library.

https://bugs.python.org/issue1294959

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This is the last part of Fedora downstream patch which was part of the commit 8510f43 (PR #18381):
https://src.fedoraproject.org/rpms/python39/blob/master/f/00102-lib64.patch

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I have questions.

• Do we really need this change?
• Who install libraries in /usr/local? Fedora packages should only install libraries in /usr, not /usr/local. Is it a convenient change for users who install modified or more recent libraries in /usr/local?
• Why only termcap requires a special change? Is it because termcap library is installed in a subdirectory (/usr/lib/termcap, not "simply" /usr/lib)

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@hroncok ran rpmdiff to compare the Fedora with the old downstream patches and with the new upstream commit 8510f43: compare the Python package with and without setup.py changes.

https://src.fedoraproject.org/rpms/python39/pull-request/31#comment-39392

In short, there is no difference. Moreover, @hroncok, @mcepl and me don't know the rationale of these setup.py changes.

I propose to drop them and wait until someone complains. If someone complains, we would know the rationale and so have a good reason to have these setup.py changes.

For readline: Fedora libreadline is already linked to libtinfo and so setup.py doesn't go up to self.compiler.find_library_file(self.lib_dirs + ['/usr/lib/termcap'], 'termcap') code path. This code is deadcode on Fedora.

For /usr/local/lib vs /usr/local/lib64: I'm not sure why /usr/local/lib is used in the first place. Fedora installs libraries in /usr, not in /usr/local. Maybe it's there for users who install dependencies without Fedora: by installed them manually in /usr/local. Again, since it's unclear how /usr/local is supposed to be used, I prefer to leave it unchanged.

I close this issue.