config: reject minversion if it's a list instead of a single string

Fixes:

src/_pytest/config/__init__.py:1071: error: Argument 1 to "Version" has incompatible type "Union[str, List[str]]"; expected "str"  [arg-type]

src/_pytest/config/__init__.py

@@ -1067,6 +1067,11 @@ class Config:
             # Imported lazily to improve start-up time.
             from packaging.version import Version
 
+            if not isinstance(minver, str):
+                raise pytest.UsageError(
+                    "%s: 'minversion' must be a single value" % self.inifile
+                )
+
             if Version(minver) > Version(pytest.__version__):
                 raise pytest.UsageError(
                     "%s: 'minversion' requires pytest-%s, actual pytest-%s'"