Fix #308: Add __deepcopy__ method to the tz.TomlTz class

[jpsca]

Without this method, the dict that results from parsing a datetime with a timezone cannot be deep-copied using the stdlib copy.deepcopy() function.

Example:

import copy
import toml

o = toml.loads("dob = 1979-05-27T07:32:00-08:00")
# {'dob': datetime.datetime(1979, 5, 27, 7, 32, tzinfo=<toml.tz.TomlTz object at 0x7f4782efb3d0>)}
copy.deepcopy(o)

This currently raises a TypeError.

[uiri]

Thank you for your pull request! Merged.