Add type hints to buck_version.py.
Add type hints to buck_version.py.

Summary: This is in preparation to fixing types when Python 3 is used.

Reviewed By: styurin

fbshipit-source-id: 0292547

LegNeato/buck,rmaz/buck,brettwooldridge/buck,shs96c/buck,romanoid/buck,brettwooldridge/buck,shs96c/buck,romanoid/buck,nguyentruongtho/buck,LegNeato/buck,Addepar/buck,shs96c/buck,Addepar/buck,JoelMarcey/buck,Addepar/buck,zpao/buck,rmaz/buck,brettwooldridge/buck,brettwooldridge/buck,kageiit/buck,shs96c/buck,brettwooldridge/buck,kageiit/buck,romanoid/buck,JoelMarcey/buck,facebook/buck,SeleniumHQ/buck,kageiit/buck,Addepar/buck,zpao/buck,brettwooldridge/buck,SeleniumHQ/buck,rmaz/buck,brettwooldridge/buck,Addepar/buck,shs96c/buck,romanoid/buck,Addepar/buck,zpao/buck,brettwooldridge/buck,Addepar/buck,romanoid/buck,LegNeato/buck,SeleniumHQ/buck,brettwooldridge/buck,LegNeato/buck,rmaz/buck,Addepar/buck,LegNeato/buck,rmaz/buck,shs96c/buck,rmaz/buck,facebook/buck,romanoid/buck,romanoid/buck,romanoid/buck,kageiit/buck,kageiit/buck,JoelMarcey/buck,SeleniumHQ/buck,shs96c/buck,JoelMarcey/buck,JoelMarcey/buck,LegNeato/buck,JoelMarcey/buck,LegNeato/buck,JoelMarcey/buck,shs96c/buck,SeleniumHQ/buck,rmaz/buck,facebook/buck,SeleniumHQ/buck,JoelMarcey/buck,romanoid/buck,romanoid/buck,Addepar/buck,SeleniumHQ/buck,LegNeato/buck,romanoid/buck,JoelMarcey/buck,zpao/buck,LegNeato/buck,brettwooldridge/buck,LegNeato/buck,zpao/buck,JoelMarcey/buck,romanoid/buck,nguyentruongtho/buck,shs96c/buck,SeleniumHQ/buck,shs96c/buck,zpao/buck,nguyentruongtho/buck,LegNeato/buck,rmaz/buck,SeleniumHQ/buck,facebook/buck,rmaz/buck,kageiit/buck,SeleniumHQ/buck,Addepar/buck,zpao/buck,shs96c/buck,brettwooldridge/buck,Addepar/buck,JoelMarcey/buck,rmaz/buck,SeleniumHQ/buck,brettwooldridge/buck,LegNeato/buck,brettwooldridge/buck,romanoid/buck,JoelMarcey/buck,rmaz/buck,shs96c/buck,SeleniumHQ/buck,shs96c/buck,nguyentruongtho/buck,nguyentruongtho/buck,facebook/buck,JoelMarcey/buck,rmaz/buck,facebook/buck,nguyentruongtho/buck,kageiit/buck,nguyentruongtho/buck,LegNeato/buck,Addepar/buck,Addepar/buck,SeleniumHQ/buck,facebook/buck,rmaz/buck