Allow environment variables to be referenced in rc files
In the process of integrating my test tool TextTest more closely with coverage.py, and trying to make use of rc files, I noticed I would either need to autogenerate them for every test run or be able to refer to environment variables in them.
I'm thinking particularly of the "data_file" and "source" settings. TextTest has its own temporary path structure where it runs tests, which it provides an environment variable for, and it would make sense to point "data_file" at that. Likewise, it knows where the source is, and sets an environment variable for that also. It would be nice to have a static coveragerc file for TextTest rather than needing to generate it each time.
This would be a one-line change: with this line
cp = configparser.ConfigParser(defaults=os.environ) }}}
in config.py inserted.