RF: refactor ascconv parse with Python ast module

The ASSCONV text header only needs its wierd double quotes fixed to be
valid Python syntax.

Fix double quotes, parse with Python ast module.  I think this should be
more robust than a custom parser.
BF: fixes for Python 3

Fixes to make tests pass for Python 3.
RF+BF+TST: refactor ElemDict to accept mapping

ElemDict claimed to accept a mapping as __init__ input, but in fact this
was broken.  Fix and test.