elpy-nav-forward/backward-block: Stop at lower indentation.

Navigating by current indentation will now stop when point would
leave the current block scope.
elpy-rpc--open: Hide RPC buffers.

RPC buffers are not really useful for users to interact with, so Elpy
now hides them from the buffer list by default, using a buffer name
with a leading space.

(Thanks to Eric Larson for the idea.)

Fixes #371
Fix some flake8 errors.
Don't error on warnings in make python-test.

Rope causes a bunch of warnings. Meh.
Do not get (too) confused by a vanishing library root.

Catch an error from Rope about a missing resource, which can
happen when the library root which existed originally suddenly
vanishes.

Also, start the Python process to check for configuration with
a default-directory of / to avoid problems when the current
directory does not exist anymore.

Fixes #353.