MNT: Explicitly point CodeCov to our coverage.xml

This way we only generate the XML once and avoid it detecting any
spurious files.

.travis.yml

@@ -150,8 +150,8 @@ script:
 after_script:
   - if [[ $TASK == "coverage" ]]; then
       pip install codecov codacy-coverage;
-      codecov -e TRAVIS_PYTHON_VERSION;
       coverage xml;
+      codecov -f coverage.xml -e TRAVIS_PYTHON_VERSION;
       python-codacy-coverage -r coverage.xml;
       ./cc-test-reporter after-build --exit-code $TRAVIS_TEST_RESULT;
     fi