include '.sql' migrations (Fixes #3045)

[grahame]

Fixes #3045

Proposed fixes:

install all migrations, include the migrations for 021 which are .sql files.

remove dead code from setup.py (the code in there made no
difference to the tarball produced by sdist.)

update MANIFEST.in to achieve what the stuff in setup.py
was trying to do.

Features:

• includes tests covering changes
• includes updated documentation
• includes user-visible changes
• includes API changes
• [X ] includes bugfix for possible backport

Please [X] all the boxes above that apply

[amercader]

@grahame can you summarize why the stuff you removed from setup.py is no longer needed? Does it actually prevent the sql files from being included?

Sorry, python packaging has always been hard to follow for me

[grahame]

@amercader: everything that was in setup.py is achieved through the MANIFEST.in file, so it was just duplication. I did a python setup.py sdist before and after my changes, and the only modification to the tarball produced was the inclusion of the 021 migration, and the documentation for migrations (which the config in setup.py was trying to include, unsuccessfully.)

[amercader]

For future reference package_data in setup.py is needed to support python setup.py bdist. Our release process uses python setup.py sdist so I think it's safe to remove for now.