Small cleanup of test_risch.py
Fix a wrong result bug in risch_integrate()

The quotient rule was mistyped in two places.
mod tracks precision of Float

Float already implements the tracking so the calculation is
passed off to Float rather than being handled in Rational.
1606: as_sum error if integral is not definite
2191: collect_const can collect Numbers
fix for Python 2.5
calculation typo
docstring and number literal edits
use Factors to process terms in collect_const