BaseTools: Avoid "is" with a literal Python 3.8 warnings

BaseTools: Avoid "is" with a literal Python 3.8 warnings

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2304

The following statement produces a SyntaxWarning with Python 3.8:

  if str(FdRegion.RegionType) is 'FILE' and self.Platform.VpdToolGuid in \
    str(FdRegion.RegionDataList):
  BaseTools/Source/Python/AutoGen/WorkspaceAutoGen.py:168: SyntaxWarning: \
    "is" with a literal. Did you mean "=="?

Change the 'is' operator by the conventional '==' comparator.

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>
Signed-off-by: Philippe Mathieu-Daude <philmd@redhat.com>