finance/beancount, finance/fava: add USE_PYTHON=noflavors

[00:01:44] Warning: (finance/beancount@py37): Error: Duplicated origin for beancount-2.1.3: finance/beancount@py37 AND finance/beancount. Rerun with -v to see which ports are depending on these.
[00:01:44] Warning: (finance/beancount@py35): Error: Duplicated origin for beancount-2.1.3: finance/beancount@py35 AND finance/beancount. Rerun with -v to see which ports are depending on these.
[00:01:44] Warning: (finance/fava@py35): Error: Duplicated origin for fava-1.9: finance/fava@py35 AND finance/fava. Rerun with -v to see which ports are depending on these.
[00:01:44] Warning: (finance/fava@py37): Error: Duplicated origin for fava-1.9: finance/fava@py37 AND finance/fava. Rerun with -v to see which ports are depending on these.

Approved by: portmgr (antoine)