move back to requiring python 3.6

portions of libanki can run on python 3.5, but media syncing will fail
so we require 3.6 when running the GUI

README.development

@@ -12,7 +12,7 @@ provide support for problems you encounter when running from source.
 
 Anki requires:
 
- - Python 3.5+
+ - Python 3.6+
  - Qt 5.9.x and a PyQT that supports it
  - mpv
  - lame

aqt/qt.py

@@ -40,3 +40,7 @@ qtpoint = QT_VERSION & 0xff
 
 if qtmajor != 5 or qtminor != 9:
     raise Exception("Anki only supports Qt 5.9.x at this time.")
+
+# GUI code assumes python 3.6+
+if sys.version_info[0] < 3 or sys.version_info[1] < 6:
+    raise Exception("Anki requires Python 3.6+")