tests: add pytest-cov .coveragerc file

When using pytest-cov this file can be used to configure the coverage report.
See https://coverage.readthedocs.io/en/latest/config.html for details.
So when you start py.test like this

  py.test --cov-config .coveragerc --cov=kallithea --cov-report xml

the settings in this file will be used.

.coveragerc

@@ -0,0 +1,14 b''
+[run]
+omit =
+    # the bin scripts are not part of the Kallithea web app
+    kallithea/bin/*
+    # we ship with no active extensions
+    kallithea/config/rcextensions/*
+    # dbmigrate and paster_commands are not part of the Kallithea web app
+    kallithea/lib/dbmigrate/*
+    kallithea/lib/paster_commands/*
+    # the tests themselves should not be part of the coverage report
+    kallithea/tests/*
+    # the scm hooks are not run in the kallithea process
+    kallithea/config/post_receive_tmpl.py
+    kallithea/config/pre_receive_tmpl.py