python: add novermin comment to tomllib import

Problem: tomllib is included on Python 3.11 and newer versions and
is loaded when available.  This can cause a vermin check failure
because the minimum required version for flux-core is 3.6.

Add a "novermin" comment to pass the vermin check.  Add a small comment
to explain code as well.

src/bindings/python/flux/util.py

@@ -30,8 +30,10 @@
 
 import yaml
 
+# tomllib added to standard library in Python 3.11
+# flux-core minimum is Python 3.6.
 try:
-    import tomllib
+    import tomllib  # novermin
 except ModuleNotFoundError:
     from flux.utils import tomli as tomllib