Formatted str should not be treated as Iterable

[okapies]

Checklist

• The bug is reproducible against the latest release or master.
• There are no similar issues or pull requests to fix it yet.

Describe the bug

As of 0.1.10, the templates use the new is_iterable function instead of is_list (#174). But now some list and details admin pages that have custom column_formatters or column_formatters_detail produce Internal Server Error.

It is because str object returned from the formatter is determined as Iterable and the formatted value is passed to _url_for_details(). It is a variation of #196.

>>> from collections.abc import Iterable
>>> isinstance("foo", Iterable)
True

Steps to reproduce the bug

No response

Expected behavior

No response

Actual behavior

No response

Debugging material

  File "/myproject/.venv/lib/python3.9/site-packages/sqladmin/templates/list.html", line 113, in block 'content'
    <a href="{{ model_admin._url_for_details(value) }}">({{ value }})</a>
  File "/myproject/.venv/lib/python3.9/site-packages/sqladmin/models.py", line 609, in _url_for_details
    pk = getattr(obj, inspect(obj).mapper.primary_key[0].name)
  File "/myproject/.venv/lib/python3.9/site-packages/sqlalchemy/inspection.py", line 71, in inspect
    raise exc.NoInspectionAvailable(
sqlalchemy.exc.NoInspectionAvailable: No inspection system is available for object of type <class 'str'>

Environment

• Ubuntu 20.04.4 LTS
• Python 3.9.12
• SQLAdmin 0.1.9

Additional context

No response