(#34) Add support for Python 3.9

[drewbanin]

Fixes #34

This PR updates Hologram's type inference code to be compatible with changes in Python 3.9. Previously, Optional types were treated as Union[_type_, None]. In Python 3.9, the Optional type is preserved directly. This PR replaces logic that checks for Union types with corresponding logic that checks for Optional types.

I smoke tested this locally, but would be great to run the dbt integration tests on Py39 while pointing to this hologram branch

[gshank]

This commit regresses the change in hologram/init.py to stop using 'best_match', which caused the test failures. This change should be undone:

-        error = next(iter(validator.iter_errors(data)), None)
+        error = jsonschema.exceptions.best_match(validator.iter_errors(data))

[drewbanin]

ooh - thanks @gshank! I was confused about the test failure for sure. I did something silly when making this branch - thanks for the quick pointer :D

[gshank]

This looks good for hologram. I've opened issue #2822 to create tox.ini targets for dbt and test with it.

[drewbanin]

hey @Sherm4nLC - this error is caused by some changes in Python v3.9 (newly released). That error is in fact fixed by this PR! If you're hitting this snag still, you can try running dbt with Python v3.8. We'll have this code change live in the next release of dbt, which should get Python v3.9 working correctly