Zxpy

- use `read1()` instead of select polling, to simplify some logic
- remove the use of `echo -n`, as that is linux-specific

Adds Python3.10 support

- Use the `select` module to do non-blocking reads from the shell process, instead of reading 1 byte at a time. Leads to significantly faster output for large streams, without any blocking.
- Use `process.communicate()` to read stdout and stderr without causing deadlock due to buffer overflow.

- Ensure the package actually works in Python 3.6+
- Make sure that `~'...'` raises an error if the process exits with a non-zero return code
- Fix an edge case in supported syntax: `[a, b, c] = ~'...'`

Now you can pass whole commands as variables inside f-strings, without quoting.

Take this piece of code:

pycon
>>> cmd = 'uname -a'
>>> ~f'{cmd}'
/bin/sh: 1: uname -a: not found


This is because `uname -a` was quoted into `'uname -a'` to avoid shell injection.

To avoid this, support for `:raw` format_spec as added:

pycon
>>> cmd = 'uname -a'
>>> ~f'{cmd:raw}'
Linux pop-os 5.11.0 [...] x86_64 GNU/Linux


Note that this shouldn't be used with external data, or this *will* expose you to shell injection.

Previously, only a few select `~'...'` statements were being printed out, rather than any bash-string without any assignments to its left. This patch version fixes printing logic.