TST: also skip datetime_NaT tests for sqlite3

- tests were failing on some combination of platform/version, see #7100, so skip for now until real solution is implemented
DOC: Update EPD link to Canopy
Merge pull request #7135 from kjordahl/doc/update_epd_to_canopy

DOC: Update EPD link to Canopy
adapt matplotlib .gitignore file
add sem to nanops
add sem to aggregate methods
add sem to groupby
add sem to docs