Udatetime

Release tested on Python 2.7, Python 3.5 and Python 3.9.

- Link shared library with libm 30
- Explicitly include <sys/timeb.h> for ftime() and struct timeb
- remove tz offset calculation on UTC timestamp

Added support for variable fractions length. A length of 1 up to 6 digits is now supported and will be normalized to microseconds.

Fixed a memory leak in the C module *rfc3339* if `_parse_date` gets called by `from_rfc3339_string`.

* Fix documentation to point out a minor issue with udatetime's timezones
* Fix DST information, return proper DST information

Thanks to apyrgio

* Fix out-of-bounds read
* Moved rfc3339 module from global namespace to udatetime. Module now named udatetime.rfc3339