fix: detect fractional-second AM/PM date formats

[nkuprins]

Closed: #1118

Purpose of the pull request

As title.

What's changed?

• Added the missing optional space to date_ptrn3b in ExcelDateFormatDetector, matching POI.
• Added a hh:mm:ss.000 AM/PM case to DateUtilsTest.test_isADateFormat_true.

Checklist

• I have read the Contributor Guide.
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.

[bengbengbalabalabeng]

LGTM.

NOTE: This change will be merged after the current release is completed. We'll follow up and push the merge forward once the release is done.