Maintainers.txt: Add continuous integration(CI) directories

Maintainers.txt: Add continuous integration(CI) directories

https://bugzilla.tianocore.org/show_bug.cgi?id=2315

Add maintainers and reviewers for the directories associated
with continuous integration steps.
* .azurepipelines
* .mergify
* .pytool

Cc: Andrew Fish <afish@apple.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>