Hi rern,
The upgrade fails with 2 errors I can see:
Retrying (Retry(total=0, connect=None, read=None, redirect=None, status=None)) after connection broken by 'SSLError(SSLError(1, u'[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:661)'),)': /simple/flask/
Could not find a version that satisfies the requirement flask (from versions: )
No matching distribution found for flask
but doing
chmod 755 /usr/lib/systemd/system/
before starting the upgrade got rid of the second error.