Opportunistically use "selectable" entry_points.

The “selectable” entry points were introduced in importlib_metadata 3.6 and Python 3.10.
While this could be solved by adding entry_points>=3.6 for python<3.10 as a dependency, python3-importib-metadata el8 package currently only have version 0.23.
Fixes deprecation warnings.
See: python/importlib_metadata#298

tmt/plugins/__init__.py

@@ -115,7 +115,13 @@ def _explore_entry_point(entry_point: str, logger: Logger) -> None:
     logger = logger.descend()
 
     try:
-        for found in entry_points()[entry_point]:
+        eps = entry_points()
+        if hasattr(eps, "select"):
+            entry_point_group = eps.select(group=entry_point)
+        else:
+            entry_point_group = eps[entry_point]
+
+        for found in entry_point_group:
             logger.debug(f"Loading plugin '{found.name}' ({found.value}).")
             found.load()