Revert BaseTools: PYTHON3 migration

This reverts commit 6693f359b3c213513c5096a06c6f67244a44dc52..
678f851312.

Python3 migration is the fundamental change. It requires every developer
to install Python3. Before this migration, the well communication and wide
verification must be done. But now, most people is not aware of this change,
and not try it. So, Python3 migration is reverted and be moved to edk2-staging
Python3 branch for the edk2 user evaluation.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Liming Gao <liming.gao@intel.com>

BaseTools/Source/Python/GenFds/VerSection.py

@@ -15,6 +15,7 @@
 ##
 # Import Modules
 #
+from __future__ import absolute_import
 from .Ffs import Ffs
 from . import Section
 import Common.LongFilePathOs as os