Aioax25

[Support for TCP-based KISS sockets](https://github.com/sjlongland/aioax25/pull/7).
Many thanks to Walter for this contribution.

- Fixed buggy APRS message serialisation (payload of
`MYCALL :message{123}None`) when reply-ACK was disabled.
- Always use fixed APRS path in replies as some digipeaters do not do AX.25
digipeating and will therefore *NOT* digipeat a message unless they see
`WIDEn-N` in the digipeater path.
- Test compatibility with Python 3.7 and 3.8.

Add support for APRS Reply-ACK message (for compatibility with Xastir).

Send KISS initialisation sequence slower to ensure the console on TNCs like the
Kantronics KPC-3 don't miss anything.

Prevent APRS digipeater from digipeating old (stale) messages.

Fix handling of APRS message ACK/REJ.