python,python3: Add PYPI_SOURCE_NAME to pypi.mk

python,python3: Add PYPI_SOURCE_NAME to pypi.mk

This adds a new (optional) variable, PYPI_SOURCE_NAME, to pypi.mk.

For some PyPi packages (e.g. aiohttp_cors, click, django-compressor),
the name of the package and the source tarball name are slightly
different (usually by capitalisation or hyphen/underscore change).

This new variable is to make this difference explicit. PYPI_NAME is
meant for the "official" package name, whereas PYPI_SOURCE_NAME is meant
for the source tarball name.

Signed-off-by: Jeffery To <redacted>