Add pandas index checks

[skrawcz]

Adds some basic index type checking to df creation

Related to issue #191, this commit is to help surface index type issues.

Specifically:

1. Warn if there are index type mismatches.
2. Require you to set your logger to debug if you want to see more details.
3. Provide a "ResultBuilder" class that uses strict index type matching so if you
   want to error on index type mismatches, this is the results builder to use.

I don't think we should build anything more custom unless there's a clear
common use case - user contributed result builders sound like an interesting idea here though.

To use the new result builder the code should be the following:

from hamilton import base, driver
strict_builder = base.StrictIndexTypePandasDataFrameResult()
adapter = base.SimplePythonGraphAdapter(strict_builder)
...
dr =  driver.Driver(config, *modules, adapter=adapter)
df = dr.execute(...)  # this will now error if index types mismatch.

Changes

• one clean up commit around imports
• one commit with the index type changes
• one commit to handle python 3.6 support and pandas index classes changing

Testing

1. Unit tests.
2. Checked things in the REPL locally against the examples in Better error handling for errors in the execute() stage #191.

Notes

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.
• Reviewers requested with the Reviewers tool ➡️

Testing checklist

Python - local testing

• python 3.6
• python 3.7
• python 3.8
• python 3.9

[elijahbenizzy]

Overall looks good -- curious on some decisions though around time-series

[elijahbenizzy]

Yeah, this is all good except the time-series stuff is confusing me.