TST: Add test util for checking for warnings

Just checks that only warnings of the specified type were produced
and works as a context manager.
add whitespace
remove whitespace
TST: Use assert_produces_warnings in test_groupby
Update RELEASE.rst
TST: Fix assert_produces_warning to reset warning filter
BUG: Simplify assert_produces_warning to get it to pass.
Remove assert produces warning entirely
Attempt 3 - totally remove the assert_produces_warning test util
TST: reset warning filter under catch_warnings