refactor: yfinance

[nikbpetrov]

Found comments to be overly verbose on main so have stripped most. Code is pretty clear.

Date setting was pretty inconsistent - 78e608d is my attempt at this - inconsistent w/ other sources that don't define an extra prop but instead pass args downstream, but that solution is UGLY here.

[nikbpetrov]

note deviation: this used to be inside the loop

[nikbpetrov]

Tests pass w/ perfect parity.

[nikbpetrov]

Rebased on top of #215

[houtanb]

Resolution fails with:

Traceback (most recent call last):
  File "/app/main.py", line 21, in <module>
    from sources.registry import SOURCES
  File "/app/sources/registry.py", line 18, in <module>
    from .yfinance import YfinanceSource
  File "/app/sources/yfinance.py", line 13, in <module>
    import yfinance as yf
ModuleNotFoundError: No module named 'yfinance'