Two cleanups for the context-managers we use:
* Use `contextlib.AbstractContextManager` if possible. This class
simply provides a default `__enter__` implementation which just
returns `self`. So use it where applicable.
Additionally, it provides an abstract `__exit__` method and thus
allows static checks for an existance of `__exit__` in the dependent
class. We might use that everywhere, but this is a separate
decision, so not included here.
* Explicitly return `None` from `__exit__`. The python docs state:
If an exception is supplied, and the method wishes to suppress
the exception (i.e., prevent it from being propagated), it
should return a true value. Otherwise, the exception will be
processed normally upon exit from this method.
That is, unless we want exceptions to be suppressed, we should
never return a `truthy` value. The python contextlib suggest using
`None` as a default return value, so lets just do that.
In particular, the explicit `return exc_type is None` that we use
has no effect at all, since it only returns `True` if no exception
was raised.
This commit cleans this up and just follows what the `contextlib`
module does and returns None everywhere (well, it returns nothing
which apparently is the same as returning `None` in python). It is
unlikely that we ever want to suppress any exceptions, anyway.
2624be92dc