setup.py: set the python_requires field

This instructs tools like pip that meson requires python 3.5 or greater,
so if one tries to install on an older version pip will fall back to an
older version of meson.

This won't fix 0.45 since it's already in the wild (unless someone wants
to delete and re-upload the version on pypi). But it should fix future
versions.

Retain the hand rolled version check, but update it to enforce our
actual requirements.

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