Restrict requirements back for gitpython (#475)

requirements.txt

@@ -3,7 +3,10 @@ torch==1.3.1
 numpy>=1.16
 torchvision==0.4.2
 scipy>=1.3.0
-gitpython
+# GitPython requirements
+gitdb==4.0.0
+gitdb2==4.0.1
+gitpython==3.0.8
 torchnet==0.0.4
 tensorflow~=1.14
 pydot==1.4.1