[3.6] bpo-29514: Check magic number for bugfix releases (#2157)

[3.6] bpo-29514: Check magic number for bugfix releases (#2157)

Add a test to check the current MAGIC_NUMBER against the
expected number for the release if the current release is
at candidate or final level. On test failure, describe to
the developer the procedure for changing the magic number.

This ensures that pre-merge CI will automatically pick up
on magic number changes in maintenance releases (and
explain why those are problematic), rather than relying on
all core developers to be aware of the implications of
such changes.