Add a config for Coverage.py (#679)

.coveragerc

@@ -0,0 +1,25 @@
+[paths]
+source =
+  proxy
+  */proxy
+  .tox/*/lib/python*/site-packages/proxy
+
+[report]
+skip_covered = true
+show_missing = true
+exclude_lines =
+    \#\s*pragma: no cover
+    ^\s*raise AssertionError\b
+    ^\s*raise NotImplementedError\b
+    ^\s*return NotImplemented\b
+    ^\s*raise$
+    ^if __name__ == ['"]__main__['"]:$
+
+[run]
+branch = true
+cover_pylib = false
+parallel = true
+relative_files = true
+source =
+  proxy
+  tests