Check PY_310_MIN instead of PY_39_MIN

Python 3.9.1 changed the `type()` of `Literal` instances to be
`_LiteralGenericAlias` instead of `_GenericAlias`.

`is_literal_type()` assumed the change took place in Python 3.9.0 and
checked for it accordingly. Because the assumption was incorrect, it
failed to detect `Literal` types in Python versions [3.9.0, 3.9.1).

This commit changes `is_literal_type()` to check `PY_310_MIN` instead of
`PY_39_MIN` to fix the issue.

mashumaro/core/meta/helpers.py

@@ -29,6 +29,7 @@
     PY_37,
     PY_38,
     PY_38_MIN,
+    PY_39,
     PY_39_MIN,
     PY_310_MIN,
     PY_311_MIN,
@@ -362,10 +363,10 @@ def is_annotated(typ: Type) -> bool:
 
 
 def is_literal(typ: Type) -> bool:
-    if PY_37 or PY_38:
+    if PY_37 or PY_38 or PY_39:
         with suppress(AttributeError):
             return is_generic(typ) and get_generic_name(typ, True) == "Literal"
-    elif PY_39_MIN:
+    elif PY_310_MIN:
         with suppress(AttributeError):
             # noinspection PyProtectedMember
             # noinspection PyUnresolvedReferences

tests/test_meta.py

@@ -94,7 +94,8 @@ def test_is_init_var_unsupported_python(mocker):
 def test_is_literal_unsupported_python(mocker):
     mocker.patch("mashumaro.core.meta.helpers.PY_37", False)
     mocker.patch("mashumaro.core.meta.helpers.PY_38", False)
-    mocker.patch("mashumaro.core.meta.helpers.PY_39_MIN", False)
+    mocker.patch("mashumaro.core.meta.helpers.PY_39", False)
+    mocker.patch("mashumaro.core.meta.helpers.PY_310_MIN", False)
     assert not is_literal(typing_extensions.Literal[1])