myst_parser is not needed to run tests.

--- sybil-9.0.0/setup.py.orig
+++ sybil-9.0.0/setup.py
@@ -30,7 +30,6 @@
         pytest=[PYTEST_VERSION_SPEC],
         test=[
             'mypy',
-            'myst_parser',
             PYTEST_VERSION_SPEC,
             'pytest-cov',
             'seedir',