Remove support for Python 3.8

[bkeryan]

• This contribution adheres to CONTRIBUTING.md.
• I've updated CHANGELOG.md if applicable.
  - [ ] I've added tests applicable for this pull request

What does this Pull Request accomplish?

Remove Python 3.8 support.

Why should this Pull Request be merged?

Fixes #647

NI's Python support policy says:

NI products that support or integrate with Python will drop official support for all Python versions that have reached their end of life EOL . This should be done at the next release of the product following the EOL date for a given Python version

Also, more of nidaqmx-python's dependencies (e.g. protobuf) have started to drop Python 3.8 support.

What testing has been done?

PR build.

[github-actions]

Test Results

    34 files   -     6      34 suites   - 6   55m 7s ⏱️ - 8m 17s
 2 382 tests ±    0   2 028 ✅ ±    0    354 💤 ±    0  0 ❌ ±0 
42 864 runs   - 7 604  36 612 ✅  - 6 512  6 252 💤  - 1 092  0 ❌ ±0 

Results for commit 2eb2505. ± Comparison against base commit 601db52.

♻️ This comment has been updated with latest results.