Allow to decide whether to use distutils or sysconfig with sysconfig._PIP_USE_SYSCONFIG

[hroncok]

(description of the change, taken from 63b5051)

Allow Python distributors to opt-out from or opt-in to the sysconfig installation scheme backend. By default, the installation scheme backend is sysconfig on Python 3.10 or later, but by setting the sysconfig._PIP_USE_SYSCONFIG the distributors can override this default: The boolean value of this private sysconfig attribute will take precedence over the default if set. This can be used for compatibility considerations as well as for opting in for more leading-edge technology if so desired. Distributors who set sysconfig._PIP_USE_SYSCONFIG = False are strongly encouraged to ensure distutils is present, e.g. it is not recommended to set this when distutils was removed from the standard library, as it will result in failures.

Fixes #10647

[pradyunsg]

One option if you want to fully test this would be to factor this expression out into a function, and the instrument the dependencies of this function.

def _should_use_sysconfig():
    ...

# This is a function for testability, but should be constant during any one run.
_USE_SYSCONFIG = _should_use_sysconfig()

[hroncok]

OK, this works as expected. Will play with the tests next week.

[pradyunsg]

Thanks @hroncok! :)

[hroncok]

Thanks for taking this.