fix issue #192 - remove six and python 2 compatability code

[lonnen]

parsimonious is all-in on python 3, so six can be removed.

Most of the changes are simple replacements, but there are a few more complicated replacements:

• catching unwrapped_exceptions was moved into the general except Exception block because python 3 does not allow except tuple[()] though python 2 did. see parsimonious/nodes.py:217

• python 3 totally changed how retrieving exception info and reraising works. The code is much simpler than was six required, involving fewer intermediary variables. see parsimonious/nodes.py:225

• six documentation suggests using the __metaclass__ property to set a metaclass, which did not work. The python 3 reference suggested using the metaclass=MyClass keyword, which works well.

$ tox passes locally for me across all versions. Travis-CI is failing to install or run anything in the python 3.10 environment, though the other environments pass

[lonnen]

I rebased these changes against master now that CI has landed and is working again, which required a force push over the branch. No changes to the patches were made as part of this force push

[lonnen]

one more force push here to catch the update that triggers CI on PRs

[lonnen]

I let this sit for a day, then self-reviewed it line-by-line. I'm satisfied with it