devShells: one global python devshell + activation via command

- this adds devShells.{system}.python
- a 'select-shell' command to switch between devshells

pkgs/scripts/select-shell.py

@@ -0,0 +1,39 @@
+import argparse
+import json
+import pathlib
+import subprocess
+import sys
+
+parser = argparse.ArgumentParser(description="Select a devshell")
+parser.add_argument("shell", help="the name of the devshell to select", nargs="?")
+parser.add_argument("--list", action="store_true", help="list available devshells")
+args = parser.parse_args()
+
+selected_shell_file = pathlib.Path(".direnv/selected-shell")
+
+if not args.list and not args.shell:
+    parser.print_help()
+    exit(0)
+if args.list:
+    flake_show = subprocess.run(
+        ["nix", "flake", "show", "--json", "--no-write-lock-file"],
+        stdout=subprocess.PIPE,
+    )
+    data = json.loads(flake_show.stdout.decode())
+    print("Available devshells:")
+    print("\n".join(data["devShells"]["x86_64-linux"].keys()))
+    exit(0)
+if selected_shell_file.exists():
+    with open(selected_shell_file) as f:
+        current_shell = f.read().strip()
+else:
+    current_shell = ""
+
+if current_shell == args.shell:
+    print(f"{args.shell} devshell already selected. No changes made.")
+    sys.exit(0)
+
+with open(selected_shell_file, "w") as f:
+    f.write(args.shell)
+
+print(f"{args.shell} devshell selected")