Replace pytest-astropy with the individual pytest plugins

[larrybradley]

The test extra now lists pytest, pytest-astropy-header, pytest-doctestplus, pytest-filter-subpackage, and pytest-remotedata in place of pytest-astropy. This fixes the intermittent py314-test-alldeps-devinfra failure in the weekly cron. The development version of pytest rejects the remote_data option destination shared by pytest-astropy (-R) and pytest-remotedata (--remote-data) when pytest-astropy is loaded first.

The devinfra tox environment and the "More architectures" cron job no longer install pytest-astropy.