Removing python 3.7 support due to xarray

[SorooshMani-NOAA]

Since the release of importlib-metadata version 5.0 the older (e.g. 0.20.2) versions of xarray that supported Python 3.7 cannot be used. Since those versions are pre-release, it's unlikely that they get fixed. An example of the issue caused by older xarray is:

ensembleperturbation/parsing/adcirc.py:704: in combine_outputs
    directory=runs_directory, file_outputs=file_data_variables, parallel=parallel,
ensembleperturbation/parsing/adcirc.py:6[47](https://github.com/noaa-ocs-modeling/EnsemblePerturbation/actions/runs/3575121349/jobs/6011281573#step:6:48): in parse_adcirc_outputs
    directory, variables=output_class.variables, parallel=parallel,
ensembleperturbation/parsing/adcirc.py:92: in read_directory
    filenames[0], drop_variables=drop_variables
/opt/hostedtoolcache/Python/3.7.15/x64/lib/python3.7/site-packages/xarray/backends/api.py:479: in open_dataset
    engine = plugins.guess_engine(filename_or_obj)
/opt/hostedtoolcache/Python/3.7.15/x64/lib/python3.7/site-packages/xarray/backends/plugins.py:110: in guess_engine
    engines = list_engines()
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

    @functools.lru_cache(maxsize=1)
    def list_engines():
>       entrypoints = entry_points().get("xarray.backends", ())
E       AttributeError: 'EntryPoints' object has no attribute 'get'

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.82%. Comparing base (b052d3f) to head (7358613).
Report is 82 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #144   +/-   ##
=======================================
  Coverage   64.82%   64.82%           
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  Files          35       35           
  Lines        3863     3863           
=======================================
  Hits         2504     2504           
  Misses       1359     1359           

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