This covers existing behaviours of `mitogen.master.scan_code_imports()` some
of which are relied on, some not, but regardless weren't tested. Notably
- Explicit relative imports return level > 0
- Imports inside `class` and `def` are excluded
- Imports inside other blocks are included
- Python 3.x prunes impossible if/else branches (previously unknown)
It also
- Decouples the test results from the implementation details of the unit test.
- Fixes a missing import
- Fixes at least one Python 2.4 incompatibility (use of with block)
Alex Willmer