Fix deprecated pipes module

[fsagbuya]

Fix pipes deprecation in Python 3.13, see PEP 594.

> nixops
Traceback (most recent call last):
  File "/nix/store/kflgqvvc23cr4s3sgma6zdcinzn24rkc-nixops-1.7-unstable-2024-02-28/bin/.nixops-wrapped", line 6, in <module>
    from nixops.__main__ import main
  File "/nix/store/kflgqvvc23cr4s3sgma6zdcinzn24rkc-nixops-1.7-unstable-2024-02-28/lib/python3.13/site-packages/nixops/__main__.py", line 35, in <module>
    from nixops.script_defs import setup_logging
  File "/nix/store/kflgqvvc23cr4s3sgma6zdcinzn24rkc-nixops-1.7-unstable-2024-02-28/lib/python3.13/site-packages/nixops/script_defs.py", line 25, in <module>
    import pipes
ModuleNotFoundError: No module named 'pipes'

[roberth]

Unrelated CI failure; merging optimistically

Thank you!