Update pyqt5 requirement from >=5.9.0 to >=5.15.11

Updates the requirements on [pyqt5](https://www.riverbankcomputing.com/software/pyqt/) to permit the latest version.

---
updated-dependencies:
- dependency-name: pyqt5
  dependency-version: 5.15.11
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

doc/requirements.txt

@@ -1,3 +1,3 @@
 sphinx==8.1.3
 sphinx-autodoc-typehints>=1.10.3
-PyQt5>=5.9.0
+PyQt5>=5.15.11