Update _ome.py

[rysk-t]

This change is for custom-date format like "%Y/%m/%d %H:%M:%S" (our nd2 files include such kinds of date format representing the acquisition date). This change might be for minor language user.

[codspeed-hq]

CodSpeed Performance Report

Merging #221 will not alter performance

_{Comparing rysk-t:main (7017236) with main (706fe8d)}

Summary

✅ 13 untouched benchmarks

[tlambert03]

thanks @rysk-t. can you give me a literal string example of the type of date you have in your file? I might actually upstream the fix to ome-types. but in the meantime, you also need to import datetime here for tests to pass

[tlambert03]

found a better solution for extracting datetime in #226

[rysk-t]

Thanks for your reply, and sorry for my lazy PR.
The example acquisiton-date string of our system is "2023/06/13 15:03:43".
I believe this string style is unique to the Japanese system. However, considering that the developer of the nd2 file (Nikon) is a Japanese company, the influence is not so small.

[tlambert03]

Thanks!

Actually nd2 is made by a small Czech company! https://www.lim.cz
(Contracted by Nikon of course)

They've been very helpful in general. I've got a better locale-independent source for the timestamp now, just dealing with some final issues with certain dates on windows. Will release a fix soon. Thanks again for your pr