Move version info around, clean up some bumpversion stuff

[beckjake]

This PR is a bunch of minor version management stuff. Most of this will be reflected in an equivalent dbt-presto PR... soon.

Moving the spark version to its own file lets us use bumpversion without too much pain and nonsense.

• Moved the spark version to a __version__.py in the spark adapter folder.

  • It would be super nice for dbt to parse this and report plugin versions in dbt --version and/or dbt debug
  • We parse this with an ugly regex

• New bumpversion config: .bumpversion-dbt.cfg

  • updates setup.py, which has the required dbt version
  • access with bumpversion --config-file .bumpversion-dbt.cfg ...

• Existing bumpversion config:

  • updates dbt/adapters/spark/version.py, which has the spark plugin version
  • access with bumpversion ... like before
  • has a new optional field, pluginpatch for issuing plugin patches when the dbt version hasn't incremented

[jtcohen6]

Everything in this PR makes sense to me. It feels a little overkill for just Spark right now, but as far as developing a framework that also works for Presto—and someday community-supported adapters, too—it's definitely worthwhile.

There are some unit test failures that are a bit unexpected. I don't see how they're related to changes you're making here, and we didn't see them in #69.

[beckjake]

Those unit tests were 0.16.0 issues - I had to copy+paste test/unit/utils.py from dbt-core, and I had to do similar for presto. It would be good to get that into somewhere common but for now we'll just live that copy+paste life.

[jtcohen6]

LGTM!

I would love if dbt --version had a way of checking dbt/adapters/*/__version__.py. One of our many TODOs when it's time to tear out the core adapters.

[beckjake]

There's a PR in active development by an external contributor that should handle this. I have high hopes for it getting into 0.17.0: dbt-labs/dbt-core#2279