eliminate warnings in the test runner

[lonnen]

getargspec() is deprecated. getfullargspec() is supposed to be a drop in replacement for most use cases, expanding the return value of getargspec() to include function annotations and keyword-only parameters. The exact behavior changed in 3.6 and again in 3.7, so if this becomes a problem look into directly calling signature(), which underlies getfullargspec(), in order to build exactly what is needed.

Commit https://github.com/erikrose/parsimonious/commit/67d92ef5b1f81ca1b43ab467fcd8822e2a6277f9 resolves the warnings captured during test runs, such that there is no longer a warnings summary or warnings listen in the overall test summary.

There remains, however, another warning that occurs outside of pytest that is displayed in the tox output, about a planned and upcoming change in setuptools.

pytest-runner is the source of the warning. It has a deprecation notice that recommends avoiding setup_requires and test_requires in setup.py. Since Parsimonious no longer supports python 2.x, we can remove pytest-runner and the associated fields of setup.py. tox.ini contains the necessary information to setup and run the test environment for Python 3.x versions. While we're there we may as well also remove the import multiprocess hack for python 2.6.

[lonnen]

force push because of a rebase needed to fix a spelling error in a commit message. The patches themselves are the same as they were

[lonnen]

self-reviewed, rebased against the latest master, and merged