Fix nitpick error on type annotation

Nitpick was failing on

    TYPE_CHECKER: ClassVar[TypeChecker]

It's not clear what to do about this. `TypeChecker` was imported
correctly from `jsonschema._types` and is noted in the doc as
`jsonschema.TypeChecker`. We can't import the `jsonschema` name at
runtime because that would be circular.

To resolve, use `typing.TYPE_CHECKING` to conditionally import
`jsonschema` at type-checking time. This avoids the circular
import but allows us to write

    TYPE_CHECKER: ClassVar[jsonschema.TypeChecker]

As a result, Sphinx correctly builds a cross-reference from the
annotation, the annotation is still accurate, and runtime behavior
is left untouched.

Author: sirosen

jsonschema/_types.py

@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import numbers
 import typing
 

jsonschema/protocols.py

@@ -7,7 +7,7 @@
 
 from __future__ import annotations
 
-from typing import Any, ClassVar, Iterator
+from typing import TYPE_CHECKING, Any, ClassVar, Iterator
 import sys
 
 # doing these imports with `try ... except ImportError` doesn't pass mypy
@@ -22,8 +22,13 @@
 else:
     from typing_extensions import Protocol, runtime_checkable
 
-from jsonschema._format import FormatChecker
-from jsonschema._types import TypeChecker
+# in order for Sphinx to resolve references accurately from type annotations,
+# it needs to see names like `jsonschema.TypeChecker`
+# therefore, only import at type-checking time (to avoid circular references),
+# but use `jsonschema` for any types which will otherwise not be resolvable
+if TYPE_CHECKING:
+    import jsonschema
+
 from jsonschema.exceptions import ValidationError
 from jsonschema.validators import RefResolver
 
@@ -72,7 +77,7 @@ class Validator(Protocol):
 
     #: A `jsonschema.TypeChecker` that will be used when validating
     #: :validator:`type` properties in JSON schemas.
-    TYPE_CHECKER: ClassVar[TypeChecker]
+    TYPE_CHECKER: ClassVar[jsonschema.TypeChecker]
 
     #: The schema that was passed in when initializing the object.
     schema: dict | bool
@@ -81,7 +86,7 @@ def __init__(
         self,
         schema: dict | bool,
         resolver: RefResolver | None = None,
-        format_checker: FormatChecker | None = None,
+        format_checker: jsonschema.FormatChecker | None = None,
     ) -> None:
         ...