Fix README example and remove unused "import os" (#76)

Fix README example and remove unused "import os" (#76)

The example code in the README no longer works with the latest version.
A similar example in
https://github.com/pretix/python-drafthorse/blob/master/tests/conftest.py
exists, and uses a Decimal for
doc.trade.settlement.monetary_summation.tax_total
instead of a tuple.
Change README example to do that as well.

Also, remove an unused import ("import os").

Bug: https://github.com/pretix/python-drafthorse/issues/66