bump minimum version of PyQt5

This fixes a problem with a recent version of MacOS.

See spesmilo#6461 (comment)
If someone insists on making ElectrumABC run with a lower version of
PyQt5, the alternative solution would be to use the same workaround as
Electron-Cash Electron-Cash@1492b1d

contrib/requirements/requirements-binaries.txt

@@ -1,4 +1,4 @@
-PyQt5>=5.12.3
+PyQt5>=5.15.2
 pycryptodomex<3.7
 websocket-client
 psutil==5.6.1