dont use exception catches for warnings; modernize xdist detection

Improvements to the test suite's integration with pytest such that the
"warnings" plugin, if manually enabled, will not interfere with the test
suite, such that third parties can enable the warnings plugin or make use
of the ``-W`` parameter and SQLAlchemy's test suite will continue to pass.
Additionally, modernized the detection of the "pytest-xdist" plugin so that
plugins can be globally disabled using PYTEST_DISABLE_PLUGIN_AUTOLOAD=1
without breaking the test suite if xdist were still installed. Warning
filters that promote deprecation warnings to errors are now localized to
SQLAlchemy-specific warnings, or within SQLAlchemy-specific sources for
general Python deprecation warnings, so that non-SQLAlchemy deprecation
warnings emitted from pytest plugins should also not impact the test suite.

Fixes: #7599
Change-Id: Ibcf09af25228d39ee5a943fda82d8a9302433726

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