Fix file-access errors with newest NetCDF4, adjust testing default settings

[Zeitsperre]

No description provided.

[Zeitsperre]

I've been running into an issue like this one: Unidata/netcdf4-python#1205. Hopefully, the workaround of ensuring that files are closed properly will address this?

[coveralls]

Pull Request Test Coverage Report for Build 4357323692

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 84.263%

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Totals
Change from base Build 4346800770:	0%
Covered Lines:	3748
Relevant Lines:	4448

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💛 - Coveralls

[Zeitsperre]

@huard

I decided to throw in the testing adjustments so that individual tests do not create multiple workers. Similar to changes in Ouranosinc/xclim@b69b867, we now have pytest-xdist disabled when running pytest and enabled when running tox.

I also modified the env vars:

• MAIN_TESTDATA_BRANCH --> RAVENPY_TESTDATA_BRANCH
• SKIP_TEST_DATA --> RAVENPY_SKIP_TEST_DATA

These env vars can be set in the main CI workflow.