Remove workarounds for older Meson versions

Removing the files() statement allows us to not use source_root(), which
breaks using libepoxy as a sub-project.

Using the python3 module is better for Windows portability.

meson.build

@@ -223,17 +223,13 @@ configure_file(input: 'epoxy.pc.in',
                install_dir: join_paths(epoxy_libdir, 'pkgconfig'))
 
 # Python
-# XXX: Depend on 0.37.1 for:
-# python = import('python3').find_python()
-python = find_program('python3', required: false)
+python = import('python3').find_python()
 if not python.found()
   python = find_program('python', required: true)
 endif
 
 # Generates the dispatch tables
-# XXX: Depend on 0.38.1 for:
-# gen_dispatch_py = files('src/gen_dispatch.py')
-gen_dispatch_py = join_paths(meson.source_root(), 'src/gen_dispatch.py')
+gen_dispatch_py = files('src/gen_dispatch.py')
 
 gl_registry = files('registry/gl.xml')
 egl_registry = files('registry/egl.xml')