Store coverage data under var/ tree.

We set the COVERAGE_FILE environment variable which controls the
output file path for the .coverage file produced by python-coverage,
and also move the mypy coverage file to that location as well.

.travis.yml

@@ -14,6 +14,7 @@ cache:
     - $HOME/node
 env:
   global:
+    - COVERAGE_FILE=var/.coverage
     - COVERALLS_PARALLEL=true
     - COVERALLS_SERVICE_NAME=travis-pro
     - COVERALLS_REPO_TOKEN=hnXUEBKsORKHc8xIENGs9JjktlTb2HKlG

tools/run-mypy

@@ -149,7 +149,7 @@ if python_files:
         if args.linecoverage_report:
             # Move the coverage report to where coveralls will look for it.
             try:
-                os.rename('var/linecoverage-report/coverage.txt', '.coverage')
+                os.rename('var/linecoverage-report/coverage.txt', 'var/.coverage')
             except OSError:
                 # maybe mypy crashed; exit with its error code
                 pass