use FieldStorage.getfirst

Switch to the more modern FieldStorage.getfirst() to work around a
problem wherein
referring to form[key].value would intermittently produce
'list' object has no attribute 'value'
for key = 'C1', form = FieldStorage(None, None,
[MiniFieldStorage('firs...'ON'), MiniFieldStorage('The-Button',
'Submit')]), ]