Allow missing SERVER_SOFTWARE in appengine checks

[pquentin]

Closes #1470

I've taken inspiration from #1470, #1471 and https://cloud.google.com/appengine/docs/standard/python/tools/using-local-server#detecting_application_runtime_environment. The tests pass, but does it fix @wittfabian's original issue? I don't know.

[codecov-io]

Codecov Report

Merging #1479 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1479   +/-   ##
=======================================
  Coverage   69.05%   69.05%           
=======================================
  Files          22       22           
  Lines        2682     2682           
=======================================
  Hits         1852     1852           
  Misses        830      830

Impacted Files	Coverage Δ
src/urllib3/contrib/_appengine_environ.py	58.82% <ø> (ø)	⬆️

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[wittfabian]

This corresponds to the original solution from #1470.

[pquentin]

@wittfabian It does! But does it help with your problem?

[wittfabian]

No, because if SERVER_SOFTWARE is not defined, both methods return False.

See: https://github.com/potatolondon/djangae/blob/master/djangae/environment.py#L50

[pquentin]

Okay, I understand now. So we want to support that, we need to make sure that having is_local_appengine return True does not break anything else. Indeed, at least in tests, contrary to djangae, we not only want to differentiate local appengine from production appengine, but we also want to detect "not running in appengine at all".