Stackprinter

Fixed
Replace deprecated distutils.LooseVersion by packaging.version

Fixed
- Degrade more gracefully in environments where the standard output streams (stdout, stderr) are not available, such as the `pythonw.exe` GUI. Concretely: 1) If stackprinter's `show()` function is called in such an environment and with default arguments, it will now return silently (doing nothing) instead of crashing. 2) the 'Traceprinter' toy now uses the built in print function (so that it doesn't try to access sys.stderr.write on import).

Added
- New kwarg `suppressed_vars` to redact certain variables, e.g. to keep secrets out of error logs

Fixed
- Allows passing `(None, None, None)` to `format_exception`
- Fixed a crashing type error that could occur in longer code scopes (e.g. in the repl)

Changed
- Disabled verbose formatting for KeyboardInterrupts by default. Call `format(..., suppressed_exceptions=None`) to enforce verbose printing even on a keyboard interrupt.

Added
- New keyword arg `suppressed_exceptions` to disable verbose formatting for certain types of exceptions (generating a standard python-like traceback instead).
- New keyword arg `line_wrap` to adjust or disable the line wrap on variable values.

(beginning of time)