Use pytest.importorskip in tests

[jrbourbeau]

This PR replaces (almost all) occurrences of try/except blocks to determine if a library is available with pytest.importorskip. I wasn't able to replace this LZMA import

zarr-python/zarr/tests/test_core.py

Lines 1944 to 1947 in 55ae9eb

	try:
	from numcodecs import LZMA
	except ImportError: # pragma: no cover
	LZMA = None

because it's used in other conditional statements in test_core.py.

Also added a skip_test_env_var utility function to zarr/tests/util.py to replace some of the boilerplate code related to skipping tests based on environment variables (e.g. ZARR_TEST_ABS)

Closes #229

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/tutorial.rst
• Changes documented in docs/release.rst
• Docs build locally (e.g., run tox -e docs)
• AppVeyor and Travis CI passes
• Test coverage is 100% (Coveralls passes)

[jakirkham]

Should we handle empty strings? If so, this might help.

Suggested change

	value = os.environ.get(name, '0')
	value = os.environ.get(name, '') or '0'

[jrbourbeau]

Hmm I'm not sure. Is an empty string a common pattern for disabling things with an env var? If so, then it's probably worth supporting here. No strong opinion from me one way or the other

[jakirkham]

It's more so that it is not a common pattern for enabling things. So the concern would be someone accidentally enabling these.

That said, let's leave it since it was already this way and we can follow-up in another issue/PR.

[jakirkham]

Thanks for working on this James! This looks much better.

As CIs install some sets of these, we may want to set the environment variables on CI. Apologies. Missed that we already do. 😅

[jakirkham]

Thanks @jrbourbeau! 😄