[requirements.txt] revert PyQt5 to v5.15.3

seeing segfaults on macOS PyInstaller compiled executables after the PyQt5 patch

Author: chrissimpkins

requirements.txt

@@ -11,7 +11,7 @@ pyinstaller-hooks-contrib==2021.1  # via pyinstaller
 https://github.com/pyinstaller/pyinstaller/archive/develop.tar.gz  # via -r requirements.in
 pyqt5-qt5==5.15.2         # via pyqt5
 pyqt5-sip==12.8.1         # via pyqt5
-pyqt5==5.15.4             # via -r requirements.in
+pyqt5==5.15.3             # via -r requirements.in
 
 # The following packages are considered to be unsafe in a requirements file:
 # setuptools