pip-requirements.txt: Add python pip requirements file

pip-requirements.txt: Add python pip requirements file

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

Add pip requirements file that is used to install the
python pip modules build from the edk2-pytool-library and
edk2-pytool-extensions repositories.

These python modules provide the extensions required to
perform EDK II Continuous Integration(CI) builds.

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: Liming Gao <liming.gao@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>