_csv.pyi: minor cleanup

[AlexWaygood]

• _csv._Writer is a confusing name for a protocol, given that there's also _csv._writer and _csv.writer. It's exactly the same as _typeshed.SupportsWrite[str], so we may as well just use that.
• The strict field of a _csv.Dialect object is documented as being a bool, not an int: https://docs.python.org/3/library/csv.html#csv.Dialect.strict
• _QuotingType should really be a union of literal types: the runtime raises an exception if we pass in anything that's not QUOTE_ALL, QUOTE_MINIMAL, QUOTE_NONE or QUOTE_NONNUMERIC. It's probably unwise to use a union of literals here right now due to Temporary moratorium on literal constants (until the end of 2022) #7258, but we should use a type alias to indicate that int isn't really the correct type here.
• _csv._reader.dialect and _csv._writer.dialect are read-only properties at runtime.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉