AVRO-2921: Strict Type Checking

[kojiromike]

This is the first pass at adding type hints across all of avro in python.

• It adds strict = true to mypy configuration, which will help ensure future changes are consistent, type-wise.
• In some cases I added # type: ignore comments to avoid having to make API breaking changes to fix type errors.
• I opted to use stub files to avoid modifying existing code where possible. (Embedded type hints seem to integrate with tooling better, so we may want to change this later.)

Jira

• My PR addresses AVRO-2921
• …and adds no dependencies

Tests

• My PR adds type hints to runtime code and tests.

Commits

• My commits all reference Jira issues in their subject lines.

Documentation

• My PR does not add new functionality.

[Fokko]

I like this, what's the status?

[kojiromike]

To be honest, I am a little stuck. It seems to me there are places where the types in avro-python aren't mathematically sound and can't be correctly described using type hints. (In particular, the subclassing of types of Schema don't always graph right.) In the long run, I think a breaking refactor will be required to make those type-able.

But I guess we can ignore those for now. The bigger issue is that mypy doesn't support recursive types, which I think severely limits the usefulness of type hinting in Avro. Avro schema is obviously suited to a recursive description of what it can serialize.

I've played with a bunch of approaches, including an arbitrary recursion-depth limit (which Python has anyway for stack recursion), but it's a bit depressing, so I took a step away from this for a while.

Mypy continues to improve, so I hope that with future improvements this will become increasingly useful.

In the mean time, I will see about fixing it up and merging it for partial advances in type hinting from avro-python.

[asosnovsky-sumologic]

@kojiromike at least having some top level type-checking is till very useful for catching errors.
Also the issue you referenced is now resolved :) python/mypy#731

[kojiromike]

@asosnovsky-sumologic Yep, I saw that and will see about getting back into this. Pretty busy in my personal life, so others are welcome to give it a shot if you can get it done faster than I can.