chore: Added mypy to noxfile

[gkevinzheng]

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• Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> 🦕

[daniel-sanche]

I see firestore adds an escape character here. Maybe that's why it's not working?

[gkevinzheng]

I found out that using s.replace instead of place_before makes it work. The escape characters were used because s.replace takes in a regex.

[daniel-sanche]

are those big undertakings? logging_v2 should already have valid types in place. And I'd say tests are optional

[gkevinzheng]

I guess big is an overexaggeration, but when I ran mypy there were 22 errors in 12 files.

[daniel-sanche]

you should also add a github action to execute mypy as a presubmit check

[parthea]

I'm going to close this PR but we will attempt to introduce mypy checks as part of the code migration to google-cloud-python using the monorepo shared CI.