Dropped `coveralls` dev-requirement in favor of using the `coverage`
package directly, and upgraded it to the latest release series (7.x).

The `line_bits` table in .coverage (SQLite DB of results, generated by
`coverage`) isn't populated. coverallsapp/coverage-reporter tries an SQL
query against the database based on this empty table, and fails because
there aren't any coverage results to upload.

We work around this by having `coverage xml` generate a file format that
the reporter can understand.

In `.coveragerc`, switched from setting the `source` option to using the
`include` option instead; this fixes incorrect relative paths being
reported to Coveralls (which breaks both source-fetching and the tree).

New release today is supposed to fix the issues with Python coverage
parsing. We will see!