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>