Update pandas requirements

[kandersolar]

Description

This PR updates the requirements spec to prohibit pandas==1.1.*. I figured there was little point in cooking up a workaround that works on all pandas versions because there were only ~6 months between 1.0.5 and 1.2.0 and it's been about a year since 1.2.0 was released.

I tested a range of pandas versions locally (windows) before opening this PR and include the results for reference:

• pandas==1.1.0: access violation/segfault
• pandas>=1.1.1,<=1.1.5: test failure
• pandas>=1.2.0: pass

Checklist

• Closes Support pandas >= 1.1.0 #82
• Added new API functions to docs/api.rst
• Clearly documented all new API functions with PEP257 and numpydoc compliant docstrings
• 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`).
• Non-API functions clearly documented with docstrings or comments as necessary
• Added tests to cover all new or modified code
• Pull request is nearly complete and ready for detailed review

[kandersolar]

This has nothing to do with the pandas change; it's because macos-latest recently changed to macos 11 instead of macos 10, and they dropped python 3.6 in macos 11. That made the 3.6 test fail here because this is the first PR since the change. See the table here: actions/runner-images#4060.

We might consider dropping python 3.6 from testing though as it is slated to reach EOL in a week or two: https://devguide.python.org/#status-of-python-branches

[cwhanse]

+1 to dropping support for python 3.6. I'm OK to do that in this PR if it simplifies the pandas requirement.

[kandersolar]

I don't think dropping 3.6 changes anything for the pandas requirement. pvanalytics's current minimum (0.24.0) supports python 3.7, so dropping 3.6 is not itself an excuse to bump our minimum and skip over the problematic 1.1.x series.

[wfvining]

Do you think we want to prohibit pandas 1.1 across the board? It's just a few functions that depend on the rolling groupby functionality. I'd kind of like to just add a guard in front of those functions that causes an error if the 1.1.x is installed (or issues a warning), but I'm not sure of the added maintenance burden on that approach.

[kandersolar]

Normally I'd also support doing a little gymnastics to not drop support unnecessarily, so I wouldn't object to some kind of function-level version check here. RdTools has its own set of pandas restrictions (link), so that's another motivator, at least to the extent that we want pvanalytics and RdTools to be used in tandem.

[cwhanse]

I say we merge this for v0.2 and add an issue to drop support for py36 and pandas<1.2 in v0.3.

[cwhanse]

Thanks @kanderso-nrel