CI: docs fail on RTD

[oberstet]

seems like mnemonic + flatbuffers is missing for the RTD build auto-triggered on releases and from master:

[autosummary] generating autosummary for: asynchronous-programming.rst, changelog.rst, contents.rst, index.rst, installation.rst, reference/autobahn.asyncio.rst, reference/autobahn.rawsocket.rst, reference/autobahn.rst, reference/autobahn.twisted.rst, reference/autobahn.util.rst, ..., reference/autobahn.websocket.rst, reference/autobahn.xbr.rst, wamp/aio-repl.rst, wamp/examples.rst, wamp/programming.rst, websocket/examples.rst, websocket/programming.rst, xbr-auth.rst, xbr-cli.rst, xbr.rst
  File "/home/docs/checkouts/readthedocs.org/user_builds/autobahn/checkouts/latest/autobahn/xbr/__init__.py", line 38, in <module>
    from mnemonic import Mnemonic
No module named 'mnemonic'Failed to import autobahn.wamp.auth.
Possible hints:
* AttributeError: module 'autobahn.wamp' has no attribute 'auth'
* ModuleNotFoundError: No module named 'cryptography'
Failed to import autobahn.wamp.message_fbs.
Possible hints:
* ModuleNotFoundError: No module named 'flatbuffers'
* AttributeError: module 'autobahn.wamp' has no attribute 'message_fbs'

https://readthedocs.org/projects/autobahn/builds/16366548/

[oberstet]

see https://github.com/crossbario/autobahn-python/pull/1540

[oberstet]

fixed via above https://autobahn.readthedocs.io/en/latest/