fix keyerror with single element list passed to Tickers

[antoniouaa]

Remade this to fix issue #899

From previous PR,

I don't see any good reason to treat single element symbol lists passed to yfinance.Tickers as something special, instead perform the pandas.concat like you would normally (with multiple tickers).

Avoids a KeyError raised here,

yfinance/yfinance/tickers.py

Line 83 in 9e2252e

self.tickers.get(symbol, {})._history = data[symbol]