Use temp .netrc file for integration tests and support NETRC environment variable

[chuckwondo]

The primary intent for this PR was to address #806. Because the changes required for #806 also produced at least partial completion of #743 and #480, it also includes the full set of changes for them as well. All 3 issues are very closely related, so it didn't make much sense to attempt to separate them out.

As a result,

• contributors may now run integration tests locally without clobbering their existing ~/.netrc file, if they have one
• because the bullet above required a means for configuring integration tests to use a temporary .netrc file location, it made sense to do so by setting a NETRC environment variable for this purpose, so users may now also set this environment variable, if they need to specify a different location for their .netrc file
• to better enable local execution of integration tests, some refactoring was required, which led to simply folding in the changes required to fix Update integration tests to use pytest conventions #743 as well

Fixes #806
Fixes #743
Fixes #480

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📚 Documentation preview 📚: https://earthaccess--809.org.readthedocs.build/en/809/

[chuckwondo]

Closing in favor of #808. I created this PR only to see if the failing integration tests from #808 would succeed when running for an internal PR (this one) rather than an external PR. The answer is yes, but the aim is to get external PRs to pass integration tests, which is what #810 is attempting to do. If and when #810 does the trick, I'll merge its changes into #808 to try to get that external PR to pass integration tests (with maintainer re-run required).