Introduce pytest command as recommended entry point

[davehunt]

This fixes #1629. We'd like to include a test to ensure backwards compatibility with the py.test command. We were thinking of adding a "py-dot-test" environment to tox.ini that could simply call py.test --version and if that succeeds we assume it's still working. We're interested in hearing any other suggestions.

[coveralls]

Coverage remained the same at 92.381% when pulling 393167b on davehunt:issue1629 into 54872e9 on pytest-dev:features.

[coveralls]

Coverage remained the same at 92.381% when pulling 393167b on davehunt:issue1629 into 54872e9 on pytest-dev:features.

[coveralls]

Coverage remained the same at 92.381% when pulling 393167b on davehunt:issue1629 into 54872e9 on pytest-dev:features.

[coveralls]

Coverage remained the same at 92.381% when pulling 393167b on davehunt:issue1629 into 54872e9 on pytest-dev:features.

[coveralls]

Coverage remained the same at 92.381% when pulling 393167b on davehunt:issue1629 into 54872e9 on pytest-dev:features.

[RonnyPfannschmidt]

looks great, waiting for the test-case to verify the existence of the entry-points, then it can be merged
another pair of eyes might be a good idea (the content is necessarily massive and repetitive)

[nicoddemus]

Well done! 😁

Tested locally on my laptop since AppVeyor is stuck!