Extensions: Specify JSON in the URL headers

This way the URL can simply be: `https://extesions.blender.org`
Later we will also add the Blender version in the header.

Internally the server will then use this information to point Blender to
the right API entry.

Related:
https://projects.blender.org/infrastructure/extensions-website/pulls/20

Pull Request: https://projects.blender.org/blender/blender-addons-contrib/pulls/24

bl_pkg/cli/blender_ext.py

@@ -489,11 +489,13 @@ def url_retrieve_to_data_iter(
     from urllib.error import ContentTooShortError
     from urllib.request import urlopen
 
-    with contextlib.closing(urlopen(
-            url,
-            data,
-            timeout=timeout_in_seconds,
-    )) as fp:
+    headers = {'Accept': 'application/json'}
+    request = urllib.request.Request(
+        url,
+        data=data,
+        headers=headers)
+
+    with contextlib.closing(urlopen(request, timeout=timeout_in_seconds)) as fp:
         headers = fp.info()
 
         size = -1