fix: generalize values for before_trading_start_minutes

[zhiyisun]

In original zipline, the before_trading_start_minutes is fixed as 8:45AM US/Eastern timezone. But that won't fit if the security market is in different location. Using (open time - 45min) and timezone info to support different timezone markets.

One tricky is exchange_calendars is moving from pytz to zoneinfo. It seems str() is the best function to convert timezone (expressed in both pytz and zoneinfo) to timezone string.