For example, see [issue
1554](https://github.com/nedbat/coveragepy/issues/1554) for the previous
misleading behavior when the exception being raised wasn't a
CoverageException.

Also, move tests to test_data.py, and finish covering the code.

* Add purge_files method to CoverageData, to allow for selective removal and update
of coverage data.

* Fix assert syntax so it's not true; this code isn't reached in the test unless it fails
and then it would have failed to fail.

* Remove trailing whitespace; did not expect this would matter on a blank line.

* Add type annotations required by mypy

---------

Co-authored-by: Stephan Deibel <sdeibel@wingware.com>

If the set of arcs is empty, skip the SQL operations.  We also need to
allow setting a file tracer for an unmeasured file, to avoid the Cython
problem whose fix caused the performance issue in the first place.

TBH, I don't know why we had to prevent file tracers on unmeasured
files.  Perhaps pytest-cov has changed to avoid the behavior that caused
problems.