Add PyQt5-sip to requirements.txt

[icota]

Otherwise (on Ubuntu 22.04) loading the plugin fails with:

Traceback (most recent call last):
  File "/home/igor/.lightning/plugins/lightning-qt/lightning-qt.py", line 13, in <module>
    import sip
ModuleNotFoundError: No module named 'sip'

[darosior]

Hello, thank you for the PR. I'll probably merge this trivial patch but FYI the plan is to migrate this repo to https://github.com/lightningd/plugins (see https://github.com/lightningd/plugins/pull/292).