CLN: pep8 clean up of sql.py
TST:  test_precision_conversion on windows fix (GH8324)
Merge pull request #8340 from jorisvandenbossche/sql-pep8

CLN: pep8 clean up of sql.py
BUG: rolling_window() properly averages weights in mean=True mode; removed scikits-timeseries dependency for testing; added further tests for rolling_window()