Update coveralls requirement from ~=1.7.0 to ~=1.8.0

Updates the requirements on [coveralls](https://github.com/coveralls-clients/coveralls-python) to permit the latest version.
- [Release notes](https://github.com/coveralls-clients/coveralls-python/releases)
- [Changelog](https://github.com/coveralls-clients/coveralls-python/blob/master/CHANGELOG.md)
- [Commits](TheKevJames/coveralls-python@1.7.0...1.8.0)

requirements-dev.txt

@@ -1,4 +1,4 @@
 doctr~=1.8.0
 pdoc3~=0.6.2
 responses~=0.10.6
-coveralls~=1.7.0
+coveralls~=1.8.0