meson: Music needs at least python 3.7

Post 3.38, Music is starting to use type checking. It relies on
Postponed Evaluation of Annotations (PEP 563) which has been released
in Python 3.7.

Closes: #451

meson.build

@@ -16,6 +16,11 @@ if not py_installation.found()
     error('No valid python3 binary found')
 endif
 
+# Python 3.7 is needed for postponed evalution of annotations
+if not py_installation.language_version().version_compare('>= 3.7')
+  error('Python 3.7 or newer is required.')
+endif
+
 # Constants
 PACKAGE_URL = 'https://wiki.gnome.org/Apps/Music'
 PACKAGE_URL_BUG = 'https://gitlab.gnome.org/GNOME/gnome-music/issues'