> Something was changed in 3.7

Yeah, Python 3.7 handles warning options differently: there is the PEP 565 and this commit:

commit 747f48e2e92390c44c72f52a1239959601cde157
Author: Victor Stinner <victor.stinner@gmail.com>
Date:   Tue Dec 12 22:59:48 2017 +0100

    bpo-32230: Set sys.warnoptions with -X dev (#4820)
    
    Rather than supporting dev mode directly in the warnings module, this
    instead adjusts the initialisation code to add an extra 'default'
    entry to sys.warnoptions when dev mode is enabled.
    
    This ensures that dev mode behaves *exactly* as if `-Wdefault` had
    been passed on the command line, including in the way it interacts
    with `sys.warnoptions`, and with other command line flags like `-bb`.
    
    Fix also bpo-20361: have -b & -bb options take precedence over any
    other warnings options.
    
    Patch written by Nick Coghlan, with minor modifications of Victor Stinner.