RTD: Fix inquiring active project versions

The `sphinx-build-compatibility` code needed to be improved where
inquiring project metadata.

The v2 API returns or started returning inactive builds (PR numbers) in
inquiries to active project versions. The v3 API works much better.

The project language will be inquired separately.

src/crate/theme/vendor/rtd_compat/extension.py

@@ -55,13 +55,26 @@ def manipulate_config(app, config):
 
     # We are using APIv2 to pull active versions, downloads and subprojects
     # because APIv3 requires a token.
+    try:
+        response_project = requests.get(
+            f"{scheme}://{production_domain}/api/v3/projects/{project_slug}/",
+            timeout=2,
+        ).json()
+        language = response_project["language"]["code"]
+    except Exception:
+        logger.warning(
+            "An error ocurred when hitting API to fetch project language. Defaulting to 'en'.",
+            exc_info=True,
+        )
+        language = "en"
+
     try:
         response_versions = requests.get(
-            f"{scheme}://{production_domain}/api/v2/version/?project__slug={project_slug}&active=true",
+            f"{scheme}://{production_domain}/api/v3/projects/{project_slug}/versions/?active=true",
             timeout=2,
         ).json()
         versions = [
-            (version["slug"], f"/{version['project']['language']}/{version['slug']}/")
+            (version["slug"], f"/{language}/{version['slug']}/")
             for version in response_versions["results"]
         ]
     except Exception: