* Skip uninstallable tags in QPY backwards compatibility tests

When a tag has been made, but the package has not yet landed on PyPI,
the QPY job fails in the environment-building step.  This is not
actually a failure of the QPY backwards-compatibility guarantees, and
it isn't the job of the QPY tests to detect a bad tag anyway.

* Query PyPI to find versions to test

The logic we actually want for versions to test is "find the versions
that _should_ be installable", rather than "try it and see"; the latter
is susceptible to silently suppressing errors.  This new form now
queries PyPI to find what versions of Qiskit are available in binary
distributions for this platform, and filters based on that.

* Include release candidates in testing

(cherry picked from commit 1344cdd)

# Conflicts:
#	test/qpy_compat/process_version.sh
#	test/qpy_compat/run_tests.sh