Issue 126: redact passwords in repr

[bbayles]

This PR updates the __repr__ method for URL objects such that the password attribute is not exposed.

    def test_to_repr_password(self):
        # Redact the password attribute from __repr__, but keep it on the object.
        urlobj = URL('https://user:password1@example.org/../something.txt')
        self.assertEqual(repr(urlobj), '<URL "https://user@example.org/something.txt">')
        self.assertEqual(urlobj.password, 'password1')
        self.assertEqual(
            str(urlobj), 'https://user:password1@example.org/something.txt'
        )

Closes #126.

[hynek]

This looks very thread-unsafe to me. 😅 __repr__ usually isn't used in hot loops, so a copy should be fine?

[bbayles]

I'll switch to copying in this PR:
#128