Update pytest test writing restriction

I think the basics sections is fairly self explanatory at this point especially with the copious amounts of examples we have. Also, if we kept it, we'd have to expand on pytest basics as well. I'd rather just point to the docs/getting started guides for each.

CONTRIBUTING.rst

@@ -322,9 +322,8 @@ Writing Tests
 Writing tests is done by adding or modifying files in folder `test`_.
 Take a look at
 `https://github.com/beetbox/beets/blob/master/test/test_template.py#L224`_
-to get a basic view on how tests are written. Despite using ``pytest``
-as a test runner, we prefer to write tests using the standard
-`unittest`_ testing framework.
+to get a basic view on how tests are written. We currently allow writing
+tests with either `unittest`_ or `pytest`_.
 
 Any tests that involve sending out network traffic e.g. an external API
 call, should be skipped normally and run under our weekly `integration
@@ -342,20 +341,6 @@ If you do this, it is also advised to create a similar test that 'mocks'
 the network call and can be run under normal circumstances by our CI and
 others. See `unittest.mock`_ for more info.
 
-Basics
-^^^^^^
-
--  Your file should contain a class derived from unittest.TestCase
--  Each method in this class which name starts with the letters *test*
-   will be executed to test functionality
--  Errors are raised with these methods:
-
-   -  ``self.assertEqual``
-   -  ``self.assertTrue``
-   -  ``self.assertFalse``
-   -  ``self.assertRaises``
-
--  For detailed information see `Python unittest`_
 -  **AVOID** using the ``start()`` and ``stop()`` methods of
    ``mock.patch``, as they require manual cleanup. Use the annotation or
    context manager forms instead.