Deprecate jsonschema.__version__.

importlib.metadata is in the standard library now, so there's
not much purpose in pre-calculating this attribute.

Author: Julian

sphinx_json_schema_spec/__init__.py

@@ -1,4 +1,5 @@
 from datetime import datetime
+from importlib import metadata
 from urllib.parse import urljoin
 import errno
 import os
@@ -8,8 +9,6 @@
 from lxml import html
 import certifi
 
-import jsonschema
-
 __version__ = "1.2.0"
 
 BASE_URL = "https://json-schema.org/draft-07/"
@@ -57,7 +56,8 @@ def fetch_or_load(spec_path):
 
     headers = {
         "User-Agent": "python-jsonschema v{} - documentation build v{}".format(
-            jsonschema.__version__, __version__,
+            metadata.version("jsonschema"),
+            __version__,
         ),
     }