Fix GitPython entry in requirements.txt

This now has different requirements in global-requirements for py2 and
py3.

TrivialFix

Change-Id: Ie7073e034641276f47c113de945971e8ed4b87ab

requirements.txt

@@ -4,7 +4,8 @@
 pbr!=2.1.0,>=2.0.0 # Apache-2.0
 docker>=2.4.2 # Apache-2.0
 Jinja2>=2.8 # BSD License (3 clause)
-GitPython>=1.0.1 # BSD License (3 clause)
+GitPython>=1.0.1,<2.1.12;python_version<'3.0'  # BSD License (3 clause)
+GitPython>=1.0.1;python_version>='3.0'  # BSD License (3 clause)
 six>=1.10.0 # MIT
 oslo.config>=5.1.0 # Apache-2.0
 oslo.utils>=3.33.0 # Apache-2.0