MinPlatformPkg: FSP Python script to python 3.x.

https://bugzilla.tianocore.org/show_bug.cgi?id=1930



Updated FSP Python script to support both 2.x and
3.x.

Test:
  . Verified with Python 2.7.12 and 3.6.6.
  . Verified tool result is the same before the change.
  . Both py -2 and py -3 built binary can boot.

Cc: Michael Kubacki <michael.a.kubacki@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Chasel Chiu <chasel.chiu@intel.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>