Add codecov support to AppVeyor and remove coveralls

scripts/upload-coverage.bat

@@ -0,0 +1,11 @@
+REM script called by AppVeyor to combine and upload coverage information to codecov
+if not defined PYTEST_NO_COVERAGE (
+    echo Prepare to upload coverage information
+    C:\Python36\Scripts\pip install codecov
+    C:\Python36\Scripts\coverage combine
+    C:\Python36\Scripts\coverage xml --ignore-errors
+    C:\Python36\Scripts\coverage report -m --ignore-errors
+    C:\Python36\Scripts\codecov --required -X gcov pycov search -f coverage.xml --flags %TOXENV:-= % windows
+) else (
+    echo Skipping coverage upload, PYTEST_NO_COVERAGE is set
+)