This error is a side effect of the implementation of the PEP 495. In your timezone, datetime(1, 1, 1, 0, 0, 0).timestamp() creates an internal timestamp with year=0. The problem is that the internal function ymd_to_ord() doesn't support year=0:

    /* This is incorrect if year <= 0; we really want the floor
     * here.  But so long as MINYEAR is 1, the smallest year this
     * can see is 1.
     */
    assert (year >= 1);

> This worked in Python 3.6.0 and before: (...)

The question is if the result was correct before?