[ftp] Add get_modified_time

[staceybeard]

Summary

Implemented get_modified_time in the FTPStorage class, from the Storage interface (via BaseStorage).

This function returns a file's last updated timestamp as a datetime value.

What's Included

• storages/backends/ftp.py — get_modified_time function with helper function _get_file_last_modified_details (modeled on _get_dir_details). This function uses MLST to get last modified data for a given file.

• tests/test_ftp.py — Unit tests for success and error cases in the new function.

Notes

• Unit tests of naive vs timezone-aware datetimes were modeled on those written by scjody in tests/test_gcloud.py.
• Tests and linting pass when running with changes from Update supported Django/Python versions and lint fixes #1545.
• From reading the specification at https://datatracker.ietf.org/doc/html/rfc3659#section-2.3, it seems that time values are always expected to be provided in UTC server-side. Because of this, this implementation assumes UTC when reading timestamps from the MLST command. This relates to concerns raised in Missing implementations of the Django interface for time metadata due to missing timezone information #1233 about knowing the timezone information for different backends.

[mschoettle]

@jschneier Any chance this can be considered to be included?