[AIRFLOW-6165] Housekeep utils.dates.date_range & add tests

[XD-DENG]

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Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-6165
  • In case you are fixing a typo in the documentation you can prepend your commit with [AIRFLOW-XXX], code changes always need a Jira issue.
  • In case you are proposing a fundamental code change, you need to create an Airflow Improvement Proposal (AIP).
  • In case you are adding a dependency, check if the license complies with the ASF 3rd Party License Policy.

Description

• Here are some details about my PR, including screenshots of any UI changes:

utils.dates.date_range has a few issues, like missing docstring, doesn't handle wrong-type input properly, and always try to run timezone.make_naive on start_date even if it's already naive (which will cause exception).

This PR addresses these issues, and add tests for utils.dates.date_range.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

Commits

• My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • All the public functions and the classes in the PR contain docstrings that explain what it does
  • If you implement backwards incompatible changes, please leave a note in the Updating.md so we can assign it to a appropriate release

[codecov-io]

Codecov Report

Merging #6720 into master will decrease coverage by 0.3%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #6720      +/-   ##
==========================================
- Coverage   83.84%   83.54%   -0.31%     
==========================================
  Files         669      669              
  Lines       37692    37694       +2     
==========================================
- Hits        31603    31491     -112     
- Misses       6089     6203     +114

Impacted Files	Coverage Δ
airflow/utils/dates.py	89.36% <100%> (+6.75%)	⬆️
airflow/operators/mysql_operator.py	100% <0%> (ø)	⬆️
airflow/operators/mysql_to_hive.py	100% <0%> (ø)	⬆️
airflow/kubernetes/volume_mount.py	44.44% <0%> (-55.56%)	⬇️
airflow/kubernetes/volume.py	52.94% <0%> (-47.06%)	⬇️
airflow/kubernetes/pod_launcher.py	45.25% <0%> (-46.72%)	⬇️
airflow/kubernetes/refresh_config.py	50.98% <0%> (-23.53%)	⬇️
...rflow/contrib/operators/kubernetes_pod_operator.py	78.2% <0%> (-20.52%)	⬇️
airflow/utils/sqlalchemy.py	93.22% <0%> (ø)	⬆️
airflow/configuration.py	89.13% <0%> (-3.63%)	⬇️
... and 4 more

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