Use `freezegun` to speed up time zone tests

[schwa]

The ftputil unit tests have some tests that run over a minute because they test the time zone handling, which is only accurate up to a minute.

The package ​freezegun is intended to help with time-sensitive unit tests by mocking calls in the date/time-related modules of the Python standard library.

Investigate if we can use freezegun to speed up ftputil's tests and if yes, use the package.