[Mypy] Add dataframe.py to mypy coverage

[sfc-gh-jrose]

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1. What GitHub issue is this PR addressing? Make sure that there is an accompanying issue to your PR.

   Fixes #NNNN

2. Fill out the following pre-review checklist:

   • [ ] I am adding a new automated test(s) to verify correctness of my new code
   • [ ] I am adding new logging messages
   • [ ] I am adding a new telemetry message
   • [ ] I am adding new credentials
   • [ ] I am adding a new dependency

3. Please describe how your code solves the related issue.

There are a lot of places in this file where attributes or variables are passed that don't have strong typing so I've added quite a few exceptions. In the future it might be useful to refactor some of our code to use Protocols instead of Unions of various types. Propagating the unions through all functions gets very complex, this is especially true for things like SelectStatement vs MockSelectStatement or Expression vs NamedExpression.