fix: adapt type hints to Python 3.9

Mostly NoneType and UnionType are only available from 3.10

coqpit/coqpit.py

@@ -11,11 +11,15 @@
 from dataclasses import Field, asdict, dataclass, fields, is_dataclass, replace
 from pathlib import Path
 from pprint import pprint
-from types import GenericAlias, NoneType, UnionType
+from types import GenericAlias
 from typing import TYPE_CHECKING, Any, Callable, Generic, Literal, TypeVar, Union, overload
 
 from typing_extensions import Self, TypeAlias, TypeGuard, TypeIs
 
+# TODO: Available from Python 3.10
+with contextlib.suppress(ImportError):
+    from types import UnionType
+
 if TYPE_CHECKING:  # pragma: no cover
     from dataclasses import _MISSING_TYPE
 
@@ -32,7 +36,7 @@ class _NoDefault(Generic[_T]):
 NoDefaultVar: TypeAlias = Union[_NoDefault[_T], _T]
 no_default: NoDefaultVar[Any] = _NoDefault()
 
-FieldType: TypeAlias = str | type | UnionType
+FieldType: TypeAlias = Union[str, type, "UnionType"]
 
 
 def _is_primitive_type(field_type: FieldType) -> TypeGuard[type]:
@@ -81,7 +85,10 @@ def _is_union(field_type: FieldType) -> TypeIs[UnionType]:
         bool: True if input type is `Union`
     """
     origin = typing.get_origin(field_type)
-    return origin is Union or origin is UnionType
+    is_union = origin is Union
+    with contextlib.suppress(NameError):
+        is_union = is_union or origin is UnionType
+    return is_union
 
 
 def _is_union_and_not_simple_optional(field_type: FieldType) -> TypeGuard[UnionType]:
@@ -98,7 +105,7 @@ def _is_union_and_not_simple_optional(field_type: FieldType) -> TypeGuard[UnionT
     """
     args = typing.get_args(field_type)
     is_python_union = _is_union(field_type)
-    if is_python_union and len(args) == 2 and NoneType in args:
+    if is_python_union and len(args) == 2 and type(None) in args:
         # This is an Optional type like `int | None`
         return False
     return is_python_union
@@ -129,7 +136,7 @@ def _is_optional_field(field_type: FieldType) -> TypeGuard[UnionType]:
     Returns:
         bool: True if the input field type is optional.
     """
-    return NoneType in typing.get_args(field_type)
+    return type(None) in typing.get_args(field_type)
 
 
 def _drop_none_type(field_type: FieldType) -> FieldType:
@@ -142,11 +149,11 @@ def _drop_none_type(field_type: FieldType) -> FieldType:
         return field_type
     origin = typing.get_origin(field_type)
     args = list(typing.get_args(field_type))
-    if NoneType in args:
-        args.remove(NoneType)
+    if type(None) in args:
+        args.remove(type(None))
     if len(args) == 1:
         return typing.cast(type, args[0])
-    return typing.cast(UnionType, GenericAlias(origin, args))
+    return typing.cast("UnionType", GenericAlias(origin, args))
 
 
 def _serialize(x: Any) -> Any:
@@ -257,7 +264,7 @@ def _deserialize_primitive_types(
         base_type = _drop_none_type(field_type)
         if base_type is not float and base_type is not int and base_type is not str and base_type is not bool:
             raise TypeError
-        base_type = typing.cast(type[int | float | str | bool], base_type)
+        base_type = typing.cast(type[Union[int, float, str, bool]], base_type)
         if x == float("inf") or x == float("-inf"):
             # if value type is inf return regardless.
             return x
@@ -297,7 +304,7 @@ def _deserialize(x: Any, field_type: FieldType) -> Any:
 
 CoqpitType: TypeAlias = MutableMapping[str, "CoqpitNestedValue"]
 CoqpitNestedValue: TypeAlias = Union["CoqpitValue", CoqpitType]
-CoqpitValue: TypeAlias = str | int | float | bool | None
+CoqpitValue: TypeAlias = Union[str, int, float, bool, None]
 
 
 # TODO: It should be possible to get rid of the next 3 `type: ignore`. At