Detect pytest-xdist instead of crashing :)
Comment and pep8 validation for TestEnv class
Improve debug of failing unit tests (TestEnv)

- Catch TypeError exception from assertAlmostEqual() since
  we can't do anything there and raise a "mismatch" message
  instead
- Display Python and Pythran results on separate lines
Fix EnvironmentError exception handling

The String conversion were incomplete, has to
generate a "tuple"-like output and not a list
when argcount > 3.
I'm unsure if the right fix wouldn't be that
BaseException should rather store the arguments
as a tuple instead of a list.