Require coverage.py older than 4.4

coverage.py 4.4 removed the path from the filename attribute in its reports.
This leads to incorrect coverage from codecov as it relies on this attribute to
find the source file. Require coverage.py older than version 4.4.

python/test_requirements.txt

@@ -7,4 +7,8 @@ PyHamcrest >= 1.8.0
 # This needs to be kept in sync with submodule checkout in
 # third_party/ycmd/third_party
 future     == 0.15.2
+# coverage.py 4.4 removed the path from the filename attribute in its reports.
+# This leads to incorrect coverage from codecov as it relies on this attribute
+# to find the source file.
+coverage   <  4.4
 codecov    >= 2.0.5