fix: Update coverage for Python 3.8 support

This requires changing the python requirement to disallow installing on
Python 4, since coverage added that requirement. Without this change,
poetry complains and fails dependency solving.

Author: kadler

pyproject.toml

@@ -30,7 +30,7 @@ classifiers = [
 ]
 
 [tool.poetry.dependencies]
-python = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
+python = ">=2.7,<3.0.0 || >=3.4.0,<4"
 
 
 [tool.poetry.dev-dependencies]
@@ -40,6 +40,7 @@ flake8 = ">=3.6.0"
 mock = "^3.0.5"
 bumpversion = "^0.5.0"
 pytest-cov = "~2.8"
+coverage = ">=4.5.4"
 sphinx = ">=1.8.4"
 sphinx-rtd-theme = "^0.4.0"