increased requested gitpython version to v3.0.8 to avoid this:

gen2 · gen2 · commit eef21a5456e9 · 2020-03-19T14:59:31.000+01:00
gitpython-developers/GitPython#983
diff --git a/requirements.yaml b/requirements.yaml
@@ -15,5 +15,5 @@ dependencies:
   - pytest=5.0.1
   - cartopy=0.17.0
   - vcrpy=2.0.1
-  - gitpython=3.0.5
-  - six=1.14.0
+  - gitpython=3.0.8
+  - six=1.14.0
