Use datetime.strptime tokens internally for parsing ISO 8601 week dates

Pull Request Checklist

Thank you for taking the time to improve Arrow! Before submitting your pull request, please check all appropriate boxes:

[ ] 🧪 Added tests for changed code.
[x] 🛠️ All tests pass when run locally (run tox or make test to find out!).
[x] 🧹 All linting checks pass when run locally (run tox -e lint or make lint to find out!).
[ ] 📚 Updated documentation for changed code.
[x] ⏩ Code is up-to-date with the master branch.

If you have any questions about your code changes or any of the points above, please submit your questions along with the pull request and we will try our best to help!

Description of Changes

Use datetime.strptime() tokens internally for parsing ISO 8601 week dates

Questions:

1) tests My code has passed python3.7, 3.8, and 3.9 tests. How should I deal with the pypy3 issue and the python3.6 issue properly?

ERROR:   pypy3: could not install deps [-rrequirements.txt]; v = InvocationError('/Users/yiransi/Documents/opensource/arrow/.tox/pypy3/bin/python -m pip install -rrequirements.txt', 1)
SKIPPED:  py36: InterpreterNotFound: python3.6
  py37: commands succeeded
  py38: commands succeeded
  py39: commands succeeded

2) linting checks Should I remove those test cases if the linting checks are only failing with the following message?

- hook id: flake8
- exit code: 1

tests/test_arrow.py:341:13: B015 Pointless comparison. This comparison does nothing but wastes CPU instructions. Remove it.
tests/test_arrow.py:349:13: B015 Pointless comparison. This comparison does nothing but wastes CPU instructions. Remove it.
tests/test_arrow.py:362:13: B015 Pointless comparison. This comparison does nothing but wastes CPU instructions. Remove it.
tests/test_arrow.py:370:13: B015 Pointless comparison. This comparison does nothing but wastes CPU instructions. Remove it.

Issue

Closes: #857

arrow-py / arrow