flake: Add PyThermostat to PYTHONPATH in devshell

For easier testing of PyThermostat code in the development shell, so
that the following is possible right in the repo root:

```
$ python -m pythermostat.plot
Channel 0 target temperature: 25.000
$ python
Python 3.11.9 (main, Apr  2 2024, 08:25:04) [GCC 13.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from pythermostat.client import Client
>>> c = Client()
>>> c
<pythermostat.client.Client object at 0x7f9a41cab190>
>>>
```

flake.nix

@@ -96,6 +96,9 @@
             numpy
             matplotlib
           ]);
+        shellHook = ''
+          export PYTHONPATH=`git rev-parse --show-toplevel`/pythermostat:$PYTHONPATH
+        '';
       };
 
       formatter.x86_64-linux = nixpkgs.legacyPackages.x86_64-linux.nixfmt-rfc-style;