The use of a backslash emits a warning in Python because the interpreter
thinks that it is the begin of an escape sequence which it does not
recognize.

Using raw strings (r') fixes this.

Prevent the interpreter to consider 'ascii charts' in the docstring as
escape sequences. Some (but not all) of the docstrings were changed to
be raw strings.

Make the test fail with an error if a warning is emitted.