Field.number(kind=...) supports any function that returns a number

For example, in our internal codebase, we have a function:

```python
def decimal_from_str(value: str, parens_negate: bool = False) -> Decimal:
    ...

amount = Field.number(kind=decimal_from_str)
```

src/klein/_form.py

@@ -12,7 +12,7 @@
     NoReturn,
     Optional,
     Sequence,
-    Type,
+    TypeVar,
     cast,
 )
 
@@ -29,6 +29,7 @@
 
 from ._app import KleinRenderable, _call
 from ._decorators import bindable
+from ._typing_compat import Protocol
 from .interfaces import (
     EarlyExit,
     IDependencyInjector,
@@ -41,6 +42,20 @@
 )
 
 
+_T = TypeVar("_T", contravariant=True)
+
+
+class _Numeric(Protocol[_T]):
+    def __float__(self) -> float:
+        ...
+
+    def __lt__(self, other: _T) -> bool:
+        ...
+
+    def __gt__(self, other: _T) -> bool:
+        ...
+
+
 class CrossSiteRequestForgery(Resource):
     """
     Cross site request forgery detected.  Request aborted.
@@ -258,9 +273,9 @@ def hidden(cls, name: str, value: str, **kw: Any) -> "Field":
     @classmethod
     def number(
         cls,
-        minimum: Optional[int] = None,
-        maximum: Optional[int] = None,
-        kind: Type = float,
+        minimum: Optional[_Numeric] = None,
+        maximum: Optional[_Numeric] = None,
+        kind: Callable[[str], _Numeric] = float,
         **kw: Any,
     ) -> "Field":
         """