Validating the appdata with appstreamcli version 0.12.2 results in the following output:

$ appstreamcli validate conf/mu.appdata.xml                                                                                                                                                   
W - mu.appdata.xml:mu.codewith.editor.desktop:29                                                                                                                                              
    Found invalid tag: '_developer_name'. Non-standard tags must be prefixed with 
    "x-".

I - mu.appdata.xml:mu.codewith.editor.desktop:7
    The component summary should not end with a "." [A simple Python editor for 
    beginner programmers.]

E - mu.appdata.xml:mu.codewith.editor.desktop:20
    The description contains an URL. This is not allowed, please use the <url/> tag 
    to share links.

Validation failed: errors: 1, warnings: 1, infos: 1

This update corrects these issues

Prefer over non-standard <x-developer_name/>