Add new checks for except (...): clauses

[Zac-HD]

When using a tuple of exception types in an except clause, it's easy for inexperienced Pythonistas to include elements that don't have any function at all. For example:

try: ...
except (Exception, TypeError): ...	 # TypeError is a subclass of Exception (builtins only)
except (MyError, MyError): ...  	 # Duplicate exception types are useless
except (MyError,): ...  			 # Should use bare name, not tuple
except (MyError, BaseException): ... # Even non-builtins subclass BaseException

This PR adds checks for all these cases, recommends the improved version (with as-clause if applicable), and thus closes #98.

CC @cooperlees

[ambv]

This is fantastic.

[ambv]

I usually just say a "one-tuple".

[Zac-HD]

Makes sense to me, but I've left it as "length-one tuple" to minimize the terminology that new programmers need to learn (right now to use the linter).

For example I have a tox config for formatters+linters that I encourage all my students to use when they're learning Python - in my experience automated feedback on what "good code" looks like can be really helpful, and the other bugbear messages are great at explaining why as well as what.

[cooperlees]

Yup, nothing to fault here. Thanks!

I can understand both statements about the one element tuple, so up to you two there :)

[Zac-HD]

Turns out that this is useful for well-linted projects too:

hypothesis\internal\validation.py:85:5: B014 Redundant exception types in except (TypeError, OverflowError, ValueError, ArithmeticError):. Write except (TypeError, ValueError, ArithmeticError):, which catches exactly the same exceptions.

[cooperlees]

Will merge and release tonight (US Pacific time)