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

[chalmerlowe]

If possible, please also remove all references to Python 3.8, since we have approval to deprecate support for that version as well as 3.7.

I did a comprehensive review of all the files in the repo to find any references to 3. since there may be conditionals such as:
< 3.9 or <= 3.9 and because there may be references to things that are no longer needed if support for a specific version of Python is dropped.

The following files have content that needs to be corrected OR reviewed for correctness AND/OR deleted.

* CONTRIBUTING.rst (multiple references, including instructions that guide users to use 3.8)
* noxfile.py (references 3.8)
* samples/snippets/noxfile.py (references 3.8)
* owlbot.py
* pytest.ini (references removing code once we drop support for 3.7 and 3.8)
* python-db-dtypes-pandas/README.rst (references supported versions of Python)
* docs/README.rst (references supported versions of Python) (probably autogenerated by sphinx
  so maybe no action needed)
* setup.py (references 3.8)
* sync-repo-settings.yaml (CI/CD checks related to 3.8)
* lint.yml (references 3.8)
* unittest.yml (references 3.8)
* test-samples-impl.sh (references using an older version of virtualenv to maintain 3.7 compatibility)
  #NOTE I think I have a version of virtualenv that will work. I will try to get that version
  (if memory serves, the latest version does not work so we need a slightly older one).
* .kokoro/samples/python3.7/common.cfg (whole directory for 3.7)
* .kokoro/samples/python3.8/common.cfg (whole directory for 3.8)
* db_dtypes/__init__.py
  * references need for an older version of pandas if using 3.7 and does a conditional check
    for that version of pandas
  * includes a warning indicating that Python 3.7 and 3.8 are deprecated and does a check
    for Python version
* samples/snippets/requirements-test.txt (references 3.7/>3.7)
* install_deps.tmpl.rst (references 3.7+)
* constraints-3.8.txt (delete)

If you have questions about a specific change, please reach out.