Stopped stripping microseconds with MySQL backend

Refs #19716.
Fixed #19716 -- Added support for microseconds with MySQL 5.6.4 and up

Thanks erik@cederstrand.dk for the report and Tim Graham for the review.
Required MySQLdb >= 1.2.5 for fractional seconds support

On MySQLdb < 1.2.5, MySQLdb returns None when fetching datetime/time
values with fractional seconds.
See https://github.com/farcepest/MySQLdb1/issues/24