Fix regex warning

[River739]

[ ] Wrote test for feature
[ ] Added changes to CHANGELOG.md
[ ] Bumped version number (delete if unneeded)

Changes proposed:
<

1. Fixed invalid regex escape sequence warning in "tests/test_formats.py" by using a raw string.
2. Made "test_date_formatter_format_value" portable by skipping the test on platforms where "time.tzset()" is not available (e.g., Windows).

[coveralls]

coverage: 94.978%. remained the same
when pulling 4f12671 on River739:fix-regex-warning
into 04a130e on data-8:master.

[adnanhemani]

Overall LGTM

[adnanhemani]

Would prefer to not put this change within this PR.