Double speed by using native traceback formatting

[Otto-AA]

In a project of mine mutmut run now takes half the time (instead of 70s now at 35s). In the case of #433 , executing pytest for some mutants now takes 0.x seconds instead of 10 seconds.

Pytest traceback formatting is apparently slow for large files*, so this is a bottleneck with the mutated source code.

This PR changes it to use the native traceback formatter, which takes practically no time.

I thought about keeping the pytest traceback formatter in debug mode, but given the huge performance issue, I think the default python traceback needs to suffice.

Fixes #433

* I did not investigate, if large files alone are enough. Probably it's about large classes, or stack traces with many variables.

[CyberiaResurrection]

As a data point, running max_children = 4, this has sped up the actual mutation part of full-up mutation runs over PyRoute from ~28,600 s (7h 56 m 40 s) to ~19,250s (5h 20m 50s) - a 48% speedup.