Added support for MDTM command in FTPFS

[atollk]

Type of changes

• Other

Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• I've added tests for new code.
• I accept that @PyFilesystem/maintainers may be pedantic in the code review.

Description

Fixes #456

Added a getmodified function to fs.base.FS which defaults to calling getinfo. In FTPFS, a check for the support of MDTM was added to provide a more efficient command for this specialized request.

Two calls to FS.getinfo(...).modified in fs.copy were replaced by calls to FS.getmodified.

[coveralls]

Coverage decreased (-0.006%) to 95.26% when pulling 4feb242 on atollk:issue_456 into ac8a91a on PyFilesystem:master.

[atollk]

Not sure how I would add tests for this. Are there any tests that run with an actual FTP server? Should I do extensive mocking to simulate a server that supports MDTM?

[atollk]

I added a test to test_ftpfs which, I think, decently covers this new feature, so I consider the PR ready to be reviewed / merged now.

[althonos]

Great work! A few fixes here and there, but the global structure is as intended 😄

[atollk]

Hi. Any update on this?

[althonos]

@atollk : sorry for the delay, I had a very intense month but I have a bit more time now.

I changed the current implementation a bit because the Info.modified approach had one caveat: it would cause an Info object to be created without the basic namespace, which it should normally always have, so I was a bit worried about the rest of the interface and that's why I didn't merge right away.

Instead, I kept the FS.getmodified method, and simply overloaded it in FTPFS. This means that code using getmodified should be faster now, and since you updated the relevant functions in fs.copy I think this will successfully fix #456 nevertheless.

[althonos]

@atollk : All clear now. I'll leave you time to object if there is anything to change, ping me when you're done reading and I'll merge.

[atollk]

Except for that one comment I made it looks good to me 👍

[althonos]

Perfect, merging then! Sorry for the delay.