Requirements: Restring PyQt5 version to >=5.12.3 and < 5.15.3

PyQt5 5.15.3 no longer ships binaries due to Qt's commercial license
shenanigans. This means if you install PyQt5 5.15.3, it just doesn't
work since the binaries are missing.  PyQt5 5.15.2 does not have this
problem, so we are stuck on 5.15.2 max version for now.

contrib/requirements/requirements-binaries.txt

@@ -1,4 +1,4 @@
-PyQt5>=5.12.3
+PyQt5<5.15.3,>=5.12.3
 pycryptodomex
 psutil
 

setup.py

@@ -16,6 +16,9 @@
 with open('contrib/requirements/requirements-hw.txt') as f:
     requirements_hw = f.read().splitlines()
 
+with open('contrib/requirements/requirements-binaries.txt') as f:
+    requirements_binaries = f.read().splitlines()
+
 version = imp.load_source('version', 'electroncash/version.py')
 
 if sys.version_info[:3] < (3, 6):
@@ -145,8 +148,8 @@ def run(self):
     install_requires=requirements,
     extras_require={
         'hardware': requirements_hw,
-        'gui': ['pyqt5'],
-        'all': requirements_hw + ['pyqt5']
+        'gui': requirements_binaries,
+        'all': requirements_hw + requirements_binaries
     },
     packages=[
         'electroncash',