bug#18843: Invalid block indentation in Python mode

[Fabián Ezequiel Gallina]

Fixed at 89ebffc in the emacs-24 branch.

`python-indent-region` region now follows saner approach, it uses
`python-indent-line` and skips special cases: namely block starts,
enders and dedenters.

The idea is that those mark the base indentation and stuff within them
could be re-indented.

I tested this approach in few large codebases and the results are quite
promising, in fact I found several bad indented stuff thanks to this.


PS: I unmerged this from #18796, it's not really the same thing.



Thanks for the report,
Fabián