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.

1 files changed, 2 insertions, 0 deletions

diff --git a/jsonschema/_types.py b/jsonschema/_types.py
index b024f7e..d0525bd 100644
--- a/jsonschema/_types.py
+++ b/jsonschema/_types.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import numbers
 import typing