Revert "run pytester subprocess when matplotlib built with pybind11 v3"

This reverts commit b66e2a9.

Author: ConorMacBride

tests/conftest.py

@@ -7,12 +7,3 @@
     @pytest.fixture
     def pytester(testdir):
         return testdir
-
-
-def pytest_configure(config):
-    # Matplotlib versions build with pybind11 3.0.0 or later
-    # encounter import issues unless run in subprocess mode.
-    # See: https://github.com/matplotlib/pytest-mpl/issues/248
-    import matplotlib
-    if Version(matplotlib.__version__) > Version("3.10.3"):
-        config.option.runpytest = "subprocess"