Use tz.gettz() instead of zoneinfo.gettz()

zoneinfo.gettz() seems to have problems (1 & 2) on system which do not install
the zoninfo tarball (e.g. Debian, Gentoo and Fedora) but rely on the system
zoneinfo files. This results in test failures (3 & 4)
tz.gettz() doesn't suffer from this problem.

1 https://github.com/dateutil/dateutil/issues/8
2 https://github.com/dateutil/dateutil/issues/11
3 https://github.com/pydata/pandas/issues/9059
4 https://github.com/pydata/pandas/issues/8639

Signed-off-by: Justin Lecher <jlec@gentoo.org>