Test code with both unicode_literals and without
First pass at resolving issues when testing with/without unicode_literals
Remove redundant test_*_unicode.py files
Get FITS warning collection working again.
Move to whitelisting
Turn on unicode literal testing in select test files.
Document the TEST_UNICODE_LITERALS tag.
Fix some tests that only fail when running in parallel.
Fix more issues with parallel testing
Document why the test fixtures are defined in conftest.py
Fix merge snafu
Remove token.
Merge pull request #1962 from mdboom/testing/add-test-unicode-literals-option-ast

Test with and without unicode literals by parsing with `ast`