Issue #372: test suite fails with Python 3.5: SyntaxError: invalid syntax f"..." (blais/beancount)

[Martin Michlmayr]

New issue 372: test suite fails with Python 3.5: SyntaxError: invalid syntax f"..."
https://bitbucket.org/blais/beancount/issues/372/test-suite-fails-with-python-35

Martin Michlmayr:

I don't know if you care (if not, feel free to close) but one test fails with Python 3.5:

```
ERROR collecting beancount/prices/sources/quandl_test.py
beancount/prices/sources/quandl_test.py:12: in <module>
from beancount.prices.sources import quandl
E File "/home/tbm/scratch/cvs/beancount/beancount/prices/sources/quandl.py", line 56
E url = f"https://www.quandl.com/api/v3/datasets/{database}/{dataset}.json"
E ^
E SyntaxError: invalid syntax
```