Yet another, hopefully final, fix to gitpython requirements (#475)

The gitdb versioning issue is resolved internally in gitpython
3.1.0, so moving to that and removing specific gitb requirements

requirements.txt

@@ -3,10 +3,7 @@ torch==1.3.1
 numpy>=1.16
 torchvision==0.4.2
 scipy>=1.3.0
-# GitPython requirements
-gitdb==4.0.0
-gitdb2==4.0.1
-gitpython==3.0.8
+gitpython==3.1.0
 torchnet==0.0.4
 tensorflow~=1.14
 pydot==1.4.1