include .coveragerc, needed by tox.ini

I noticed that archlinux has an identical coveragerc and wondered why.

Then noticed that our tox.ini (which we bundle into the pypi package)
needs .coveragerc, but we did not bundle it until now.

(cherry picked from commit c79156caf50fa2606bda0563e03bf3a93b23229f)

MANIFEST.in

@@ -1,7 +1,7 @@
 # stuff we need to include into the sdist is handled automatically by
 # setuptools_scm - it includes all git-committed files.
 # but we want to exclude some committed files/dirs not needed in the sdist:
-exclude .coafile .coveragerc .editorconfig .gitattributes .gitignore .mailmap .travis.yml Vagrantfile
+exclude .coafile .editorconfig .gitattributes .gitignore .mailmap .travis.yml Vagrantfile
 prune .travis
 prune .github
 include src/borg/platform/darwin.c src/borg/platform/freebsd.c src/borg/platform/linux.c src/borg/platform/posix.c