chore: ensure mypy passes

[crwilcox]

related to #447

mypy -p google.cloud.bigtable --no-incremental
Success: no issues found in 17 source files

For bigtable, specifying a subfolder fails, but package works fine. Ignoring for now

❯ ./.nox/unit-3-10/bin/mypy google/cloud/bigtable
google/cloud/bigtable/__init__.py: error: Source file found twice under different module names: "cloud.bigtable.client" and "google.cloud.bigtable.client"
Found 1 error in 1 file (errors prevented further checking)

[tswast]

Do you want to add a nox session for this?

[tseaver]

Shouldn't we be adding a mypy nox session?

[crwilcox]

This can merge, but bug should have a comment added that we want to evaluate mypy against google and tests (comment in code)

This PR needs to merge to remove a class of errors from generated code https://www.github.com/googleapis/gapic-generator-python/issues/1026

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[tseaver]

@googlebot I consent

[tseaver]

@googlebot I consent.

[crwilcox]

Seems a small coverage gap

google/cloud/bigtable/table.py                                                                 283      0     80      1    99%   64->63

[tseaver]

65a472b closes that gap.