This line was added to fix a problem with Python 2 [1,2] in Homebrew
circa 2015 via b979cea. That fix is no longer needed in 2022.

[1]: https://web.archive.org/web/20190629053857/https://github.com/Homebrew/homebrew-python/issues/187
[2]: https://web.archive.org/web/20200221230523/https://bugs.python.org/issue22269

This test was writing data into the site-packages directory if the tests
were run from an installed copy of Cantera.

In some cases, Python can issue a warning about incompatible ABI
versions on macOS when Python is compiled with one target and Cantera is
compiled with another. This change pulls the configuration value from
Python if the full Python package is being built, if it isn't set by an
environment variable.

This option was added to support Python 2/3 with Homebrew on Mac OS X.
As Python 2 is no longer supported, this line is no longer needed.

The Windows binary MSI distribution is deprecated by Python. The
replacement is wheels which are now working.

If git cannot be found by subprocess, it raises a FileNotFoundError.

Setuptools versions greater than 60.0.0 deprecate using setup.py install
commands, in preference for PEP 517/518 build backends/frontends. In
this case, we're using pip as the frontend and setuptools as the
backend.

This also uses pip to determine the installation location for the
module. It uses pip's internal API which is subject to change. However,
pip does not expose an external API to get this information, and since
Pip is the one determining where files will be installed, this seems the
most reasonable approach for now.

When using pip as the installer, setting the default python_prefix equal
to the prefix configuration value is no longer necessary. By removing
this setting, we respect the decision of the Linux distro maintainers as
to where Pip should install packages, since Pip reads this information
from the CPython configuration. On macOS, the system Python is locked at
2.7, so isn't supported anyways. On Windows, this was already the
default.

There are no longer any applications installed with Cantera.

This change mirrors the one for the Cython interface. Pip is used as
the build frontend, setuptools is used as the backend. A wheel file is
built during the 'scons build' phase to ensure that all the machinery is
working properly; this wheel is not used further.

We need these here to copy the files into the sdist.

With OpenBLAS on macOS, this test has a relative error of 1.06e-8, just
barely exceeding the original tolerance.

Since Python builds are done in the build directory, we don't need to
clean up the interfaces folder anymore.

We're dropping Python 3.6, Windows builds need wheel installed, and the
GitHub Actions macOS runners are notoriously slow.

The Sourceforge "pilotfibre" domain has an expired SSL certificate and
doesn't seem to be maintained anymore.

If the prefix is set, the Python package should be installed there.

The default_prefix variable needs to always be set. This also fixes
backslash-separated paths by replacing with forward slash. Although this
is most likely to occur on Windows, it's not platform specific.

On Windows, normpath turns forward slashes into backslashes. We want to
avoid that because paths on Windows that have backslash turn into
escapes that break things.

Hopefully move this after all path manipulations happen.