split qt requirements out again

PyQt didn't used to split WebEngine out separately, and that can
cause conflicts with distro-provided packages.

README.development

@@ -21,6 +21,9 @@ To start, make sure you have the following installed:
 
 If the distro you are using has PyQt5 installed, make sure you have the PyQt5
 WebEngine module and development tools (eg pyqt5-dev-tools) installed as well.
+Otherwise, run:
+
+$ pip install -r requirements.qt
 
 Then to run the development version:
 

requirements.qt

@@ -0,0 +1,2 @@
+pyqt5
+pyqtwebengine

requirements.txt

@@ -8,5 +8,3 @@ jsonschema
 psutil; sys_platform == "win32"
 distro; sys_platform != "win32" and sys_platform != "darwin"
 typing
-pyqt5
-pyqtwebengine