verification/dot2: Improve dot parser robustness

This patch makes the dot parser used by dot2c and dot2k slightly more
robust, namely:
* allows parsing files with the gv extension (GraphViz)
* correctly parses edges with any indentation
    * used to work only with a single character (e.g. '\t')
Additionally it fixes a couple of warnings reported by pylint such as
wrong indentation and comparison to False instead of `not ...`

Link: https://lore.kernel.org/20241017064238.41394-2-gmonaco@redhat.com


Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>