Upgrade codecov dependency (#914) (#918)

codecov==2.1.12 has been removed from PyPI but developers uploaded
newer version 2.1.13 for those who still need it - like nailgun does.

More info about the change:
https://about.codecov.io/blog/message-regarding-the-pypi-package/

(cherry picked from commit a3ab4e0)

Co-authored-by: Pavel Novotny <pnovotny@users.noreply.github.com>

requirements-dev.txt

@@ -16,4 +16,4 @@ wheel
 twine
 
 # For code coverage
-codecov==2.1.12
+codecov==2.1.13