plugin upload, XEP-0363: code modernisation + preparation for extension:

- use of async/await syntax
- fileUpload's options are now serialised, allowing non string values
- (XEP-0363) Slot is now a dataclass, so it can be modified by other plugins
- (XEP-0363) Moved SSL related code to the new tools.web module
- (XEP-0363) added `XEP-0363_upload_size` and `XEP-0363_upload` trigger points
- a Deferred is not used anymore for `progress_id`, the value is directly returned

setup.py

@@ -45,6 +45,7 @@ install_requires = [
     'sat_tmp >= 0.7.0a4',
     'shortuuid',
     'twisted[tls] >= 19.7.0',
+    'treq',
     'urwid >= 1.2.0',
     'urwid-satext >= 0.7.0a2',
     'wokkel >= 0.7.1',