Simplify code

Keeps the "happy path" on the function's base indentation level.

Avoids carrying status and return value variables, pre-filled in
anticipation of an error, along.

Avoids explicit setting of JSON content type, handles serialization to
JSON.

Merges duplicated code.

https://python-markdown.github.io/extensions/extra/

To get <pre> tags on fenced code

ref https://matrix.org/docs/spec/client_server/r0.6.0#m-room-message-msgtypes

And the alpine py3-pycryptodome package installs pycryptodomex, which is
not compatible with matrix-nio.

Thanks a lot @djanos !