fix(ci): cap anyio <4.7 for prefect 2.20 compatibility

prefect 2.20.25 (final 2.x, no backport) GatherTaskGroup doesn't implement
anyio's create_task, which anyio made abstract in newer 4.x. Hold anyio to
the last verified-good 4.6 line so the flow engine can instantiate task
groups. Verified: tox -e test now 30 passed / 21 skipped.

Author: simontaurus

setup.cfg

@@ -100,6 +100,12 @@ UI =
     pysimplegui
 workflow =
     prefect>=2.20.25,<3.0
+    # prefect 2.20.25 is the final 2.x release (no backports). Its
+    # GatherTaskGroup subclasses anyio's TaskGroup but doesn't implement
+    # create_task, which newer anyio 4.x made abstract (broken on 4.14,
+    # verified good on 4.6.2); hold anyio to the last verified-good 4.6 line
+    # so the prefect-2.x flow engine can still instantiate task groups.
+    anyio>=4.4.0,<4.7
 tutorial =
     %(dataimport)s
 all =