Adds a code coverage report to pytest output.

This would be something like the following, after the test results:

```
Name                            Stmts   Miss  Cover
---------------------------------------------------
src/__init__                        0      0   100%
src/chatexchange/__init__           0      0   100%
src/chatexchange/asyncwrapper     118    118     0%
src/chatexchange/browser          128     39    70%
src/chatexchange/wrapper          122     24    80%
---------------------------------------------------
TOTAL                             368    181    51%
```
Merge branch 'coverage' into test