Upgrade devel/py-mock from 2.0.0 to 3.0.5 and fix "make test" target

Version 3.0.5 includes multiple bugfixes and such for the backport library
that will aid in writing tests where the environment doesn't contain
unittest.mock.

In order to ensure that the code actually works, I needed to run the tests.
This required working around https://github.com/testing-cabal/mock/issues/469
by pulling the sources from the GitHub project that correspond with the
release tag, as the source tarball tagged for the release doesn't contain the
tests (prior to this change it would run 0 tests with make test, because
there weren't any).

Remove an unnecessary build dependency while here: pbr is no longer needed to
build the package.

Add py-pytest to TEST_DEPENDS, as running the tests as currently done via
distutils doesn't work due to how the tests are enumerated.

Conversely, remove the devel/py-tox dependency: it was never installed before
due to a typo in the variable name and it adds a lot of unnecessary
dependencies that aren't required when testing the port with make test. This
helps keep the footprint down, which helps others upgrade the devel/py-mock
port by itself.

Address portlint issues while here.

The changelog for the project can be found here on GitHub.

PR: 241780
Approved by: jessekempf@gmail.com (maintainer timeout), woodsdb (ports)
Discussed with: bdrewery, Vladimir.Chukharev@gmail.com
Differential Revision: https://reviews.freebsd.org/D21654