scipy 1.11 compatibility

[kandersolar]

• [ ] Closes #xxx
• [ ] Added tests to cover all new or modified code.
• [ ] Clearly documented all new API functions with PEP257 and numpydoc compliant docstrings.
• [ ] Added new API functions to docs/api.rst.
• [ ] Non-API functions clearly documented with docstrings or comments as necessary.
• Adds description and name entries in the appropriate "what's new" file
  in docs/whatsnew
  for all changes. Includes link to the GitHub Issue with :issue:`num`
  or this Pull Request with :pull:`num`. Includes contributor name
  and/or GitHub username (link with :ghuser:`user`).
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels and Milestone are assigned to the Pull Request and linked Issue.

The tests are failing in #186 due to unrelated changes in scipy (see the 1.11.0 release notes):

In scipy.stats.mode, the default value of keepdims is now False, and support for non-numeric input has been removed.

In this line of code, the .mode(...).mode is now returning a scalar, so trying to index with [0] fails:

pvanalytics/pvanalytics/quality/time.py

Line 181 in 89d3c63

lambda shifted_period: stats.mode(shifted_period).mode[0]

This PR switches away from scipy.stats.mode, which has too inconsistent an interface across all versions to be easily used here, and instead just uses pd.Series.mode.

[kperrynrel]

Ok, I just took the two changes you applied here @kandersolar, and added them to #186 to see if we'd be passing and it looks like it is! LGTM, thank you for jumping on this so quickly :)

[kperrynrel]

Tested on #186 and passed!