Support isodate ending with Z

[dalito]

The PR switches to using isodate package for isodate parsing. It is already installed with LinkML since it is a rdflib dependency.

Todo (maybe):

• restrict the use of the isodate package to Python < 3.11. Starting with Python 3.11 the standard library´s datetime.datetime.fromisoformat is able parse isodates ending with "Z" correctly.

Closes linkml/linkml#2096
Closes linkml/linkml#629

[codecov]

Codecov Report

Attention: Patch coverage is 81.81818% with 6 lines in your changes missing coverage. Please review.

Project coverage is 63.46%. Comparing base (1b5e9f7) to head (7aad389).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
linkml_runtime/utils/metamodelcore.py	81.81%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #357      +/-   ##
==========================================
+ Coverage   63.43%   63.46%   +0.02%     
==========================================
  Files          63       63              
  Lines        8982     8989       +7     
  Branches     2574     2578       +4     
==========================================
+ Hits         5698     5705       +7     
- Misses       2665     2667       +2     
+ Partials      619      617       -2     

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