Fix bugs in Color.from_cocoa and from_legacy_trigger

[mkusaka]

Summary

Three bugs in color.py, all introduced in bda9aec when pyobjc was made optional:

• MissingDependency is raised but never defined — added as a subclass of ImportError
• int(str) passes the str type instead of the parameter s — fixed to int(s)
• Return type annotation ('Color', 'Color') is invalid syntax for mypy — fixed to typing.Tuple[Optional[Color], Optional[Color]]

Examples

Color.from_cocoa raises NameError instead of a meaningful error when pyobjc is not installed:

from iterm2.color import Color

Color.from_cocoa("dGVzdA==")
# NameError: name 'MissingDependency' is not defined

Color.from_legacy_trigger always raises TypeError because int(str) passes the type object:

from iterm2.color import Color

Color.from_legacy_trigger("0")
# TypeError: int() argument must be a string, a bytes-like object or a real number, not 'type'

[gnachman]

Thanks!