[builder.names] set empty weight string if 'Regular'

[anthrotype]

same as we do for the 'width'.

Fixes #300

[schriftgestalt]

I solution is to put all pieces in an OrderedSet and remove "Regular" if it has more than 1 elements. Because you normally what to keep the "Regular" if there are not other elements?

[anthrotype]

yeah, but that is already handled by the build_style_name function

def build_style_name(width='', weight='', custom='', is_italic=False):
    """Build style name from width, weight, and custom style strings
    and whether the style is italic.
    """

    return ' '.join(
        s for s in (custom, width, weight, 'Italic' if is_italic else '') if s
    ) or 'Regular'

[anthrotype]

I could bake the logic of "treat the string 'Regular' as a special one but only for the width and weight attributes" inside the build_style_name function itself.
However, I prefer to leave it to the caller to pass correct input (and that is what we were already doing for the width -- where Regular was special-cased by the caller - but not for the weight).
Garbage in, garbage out.