At some point the defaults for pyyaml were switched to safe parsing mode, so
that the previous arbitrary Python YAML tags like
"!!python/object/apply:datetime.date [2017, 9, 1]" no longer work. A better way
is to define our own explicit constructors to avoid unsafe mode.
The current importers trim lots of extraneous symbols and whitespace from
currency strings before passing them to Decimal(). This function takes care
of all that in a single place.
