Allow affiliated packages to generate their own cython_version.py module.
Use .utils._compiler instead of astropy.utils._compiler to force affilated packages to report the compiler they were built with, not the compiler Astropy was built with.  This will require an update the package-template to include support for this optional feature.  The instructions for setting up the package should include removing this if the package does not include any compiled C extensions (in which case it's superfluous).
Merge pull request #1198 from iguananaut/issue-1198

Make ``cython_version`` logic affiliated package-friendly