textproc/py-mkdocstrings-{crystal,python-legacy}: Adjust USE_PYTHON

* Since c0c720d7eb the dependency textproc/py-mkdocs-autorefs requires
  at least Python version 3.10 or newer and packages of this port for
  older Python versions can no longer be build, thus set USE_PYTHON
  accordingly.

textproc/py-mkdocstrings-crystal/Makefile

@@ -20,7 +20,7 @@ RUN_DEPENDS=	${PYTHON_PKGNAMEPREFIX}markdown-callouts>=0.1.0:textproc/py-markdow
 # textproc/py-mkdocstrings belongs to RUN_DEPENDS as well but would cause
 # a circular dependency (via py-mkdocstrings' CRYSTAL option)
 
-USES=		python
+USES=		python:3.9+ # textproc/py-mkdocs-autorefs requires Python 3.9+
 USE_PYTHON=	autoplist pep517
 
 NO_ARCH=	yes

textproc/py-mkdocstrings-python-legacy/Makefile

@@ -21,7 +21,7 @@ TEST_DEPENDS=	${PYTHON_PKGNAMEPREFIX}mkdocs-material>0:textproc/py-mkdocs-materi
 # a circular dependency (via py-mkdocstrings' PYTHONLEGACY option).  It was
 # therefore # moved to TEST_DEPENDS to at least allow the test suite to run.
 
-USES=		python
+USES=		python:3.9+ # textproc/py-mkdocs-autorefs requires Python 3.9+
 USE_PYTHON=	autoplist pep517 pytest
 
 TEST_ENV=	PYTHONPATH=${STAGEDIR}${PYTHONPREFIX_SITELIBDIR}