deps: Update mininum support to Python 3.9, Pandas 1.5.0, Numpy 1.20.3

[chelsea-lin]

#284 introduce the ArrowExtensionArray dependency, which requires pandas version 1.5.0 and above. The pandas 1.5.0+ is only compatible with Python 3.8+ and NumPy 1.20.3+.

• Fixes internal bug b/312728178 (design doc: go/bf-json)
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes internal bug b/312728178 🦕

[tswast]

Looks like unit (3.7) is still marked as required. Could you send a PR creating a sync-repo-setting.yml first? The if one isn't created then, this is used: https://github.com/googleapis/repo-automation-bots/blob/19bb823389a77e22d00371cb53ecadab4a58ab9e/packages/sync-repo-settings/src/required-checks.json#L50

[tswast]

Let's update https://github.com/googleapis/python-db-dtypes-pandas/blob/main/.github/workflows/unittest.yml to drop 3.7 too.

Might need to exclude that file in https://github.com/googleapis/python-db-dtypes-pandas/blob/main/owlbot.py until we drop Python 3.7 google-wide.

[Linchin]

@chalmerlowe has been working on an effort to drop support for python 3.7 and 3.8 (see internal doc go/drop-python-37). We likely will need to drop the support in sync with that.

[tswast]

Since we are removing this, please update constraints-3.9.txt to include all the minimum versions from setup.py