make 'import serial' dependency sometimes optional

[thejoshwolfe]

I don't have a tty device, only a hidraw device. I imagine many people only have 1 device they want to support, so the tty-specific dependency doesn't apply to some people (including me).

This PR prints a warning on failure to import serial instead of exiting with an error, and the warning explains what's going to work and not work.

Here's what it looks like with a hidraw device without and with the serial system dependency:

josh@nixos:~/dev/temper$ ./temper.py 
WARNING: Cannot import "serial" for tty devices. Only hidraw devices will work. Please sudo apt-get install python3-serial
Bus 003 Dev 080 3553:a001 TEMPer1F_V4.1 23.43C 74.17F - - - -
josh@nixos:~/dev/temper$ nix-run -p python312Packages.pyserial -- ./temper.py 
Bus 003 Dev 080 3553:a001 TEMPer1F_V4.1 23.43C 74.17F - - - -

for a tty device, the situation would look something like this: (I tested this by hacking the code to artificially hit this codepath because i don't have a tty device.)

josh@nixos:~/dev/temper$ ./temper.py 
WARNING: Cannot import "serial" for tty devices. Only hidraw devices will work. Please sudo apt-get install python3-serial
Traceback (most recent call last):
  File "/home/josh/dev/temper/./temper.py", line 497, in <module>
    main()
  File "/home/josh/dev/temper/./temper.py", line 494, in main
    sys.exit(temper.main())
             ^^^^^^^^^^^^^
  File "/home/josh/dev/temper/./temper.py", line 487, in main
    results = self.read(args.verbose)
              ^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/josh/dev/temper/./temper.py", line 404, in read
    results.append({ **info, **usbread.read() })
                               ^^^^^^^^^^^^^^
  File "/home/josh/dev/temper/./temper.py", line 328, in read
    return self._read_serial(self.device)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/josh/dev/temper/./temper.py", line 281, in _read_serial
    import serial
ModuleNotFoundError: No module named 'serial'