Get yahoo price series only when price is not None

[mrlimacz]

When there is a bank holiday, no pricing data is available. In case of AAPL (https://finance.yahoo.com/quote/AAPL/history?p=AAPL, see 2023-07-04) timeseries returned by an API call simply skips this particular date. However, securities traded in Warsaw Stock Exchange (e. g. PKO.WA, https://finance.yahoo.com/quote/PKO.WA/history?p=PKO.WA, see 2023-11-01), return timeseries with None as a price for a timestamp of a bank holiday. This causes an exception to by thrown.

This PR simply filters out onle these timestamps with non-None price.

[rizaon]

I recently hit an issue that was fixed by this PR. This is a redacted stack trace of my issue.

Traceback (most recent call last):
  File "/Users/redacted/venv/bin/bean-price", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/Users/redacted/venv/lib/python3.11/site-packages/beanprice/price.py", line 854, in main
    price_entries = sorted(price_entries, key=lambda e: e.currency)
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/Cellar/python@3.11/3.11.6/Frameworks/Python.framework/Versions/3.11/lib/python3.11/concurrent/futures/_base.py", line 619, in result_iterator
    yield _result_or_cancel(fs.pop())
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/Cellar/python@3.11/3.11.6/Frameworks/Python.framework/Versions/3.11/lib/python3.11/concurrent/futures/_base.py", line 317, in _result_or_cancel
    return fut.result(timeout)
           ^^^^^^^^^^^^^^^^^^^
  File "/usr/local/Cellar/python@3.11/3.11.6/Frameworks/Python.framework/Versions/3.11/lib/python3.11/concurrent/futures/_base.py", line 456, in result
    return self.__get_result()
           ^^^^^^^^^^^^^^^^^^^
  File "/usr/local/Cellar/python@3.11/3.11.6/Frameworks/Python.framework/Versions/3.11/lib/python3.11/concurrent/futures/_base.py", line 401, in __get_result
    raise self._exception
  File "/usr/local/Cellar/python@3.11/3.11.6/Frameworks/Python.framework/Versions/3.11/lib/python3.11/concurrent/futures/thread.py", line 58, in run
    result = self.fn(*self.args, **self.kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/redacted/venv/lib/python3.11/site-packages/beanprice/price.py", line 592, in fetch_price
    srcprice = fetch_cached_price(source, psource.symbol, dprice.date)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/redacted/venv/lib/python3.11/site-packages/beanprice/price.py", line 498, in fetch_cached_price
    source.get_historical_price(symbol, time))
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/redacted/venv/lib/python3.11/site-packages/beanprice/sources/yahoo.py", line 157, in get_historical_price
    series, currency = get_price_series(ticker, time - timedelta(days=5), time)
                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/redacted/venv/lib/python3.11/site-packages/beanprice/sources/yahoo.py", line 102, in get_price_series
    series = [(datetime.fromtimestamp(timestamp, tz=tzone), Decimal(price))
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/redacted/venv/lib/python3.11/site-packages/beanprice/sources/yahoo.py", line 102, in <listcomp>
    series = [(datetime.fromtimestamp(timestamp, tz=tzone), Decimal(price))
                                                            ^^^^^^^^^^^^^^

@blais can we merge this PR please?

[Zhong-z]

Just realised this PR as my PR that fixes the same issue got conflicts 😄
#79. Should've tagged the author earlier.

Perhaps I can cherry pick the tests and open a new PR instead? @blais

[blais]

SGTM