Add property-based fuzz test

A test is added that runs mccabe against plenty of random yet valid
Python source code. Right now, the test does not do much - it only tests
whether mccabe accepts the code. Already this is an achievement because
now weird edge cases are covered.

Having this test suite integrated will also allow to build up on that.
The author of Hypothesis claimed to work on a coverage guided test
runner. Also more general properties to hold on any valid source code
now can be tested.
3 files changed