ci: Add missing coverage requirements and configuration

This commit fixes error like this one:

  ERROR: usage: setup.py [options] [file_or_dir] [file_or_dir] [...]
  setup.py: error: unrecognized arguments: --cov --cov-report

Author: jcfr

.coveragerc

@@ -0,0 +1,7 @@
+[run]
+branch = True
+include = castxml/*.py
+omit = castxml/_version.py
+
+[xml]
+output = tests/coverage.xml

requirements-dev.txt

@@ -1,6 +1,8 @@
 cmake
+coverage>=4.2
 flake8>=3.0.4
 pytest>=4.5.0
+pytest-cov>=2.4.0
 pytest-runner>=5.1
 pytest-virtualenv>=1.7.0
 scikit-build>=0.10.0