Stac command crashes when installed in clean venv without [validate] option

[matthewscottgordon]

Describe the bug
The test_data data uses the requests library but it is not listed as a required dependency. If requests is not installed, stac crashes while loading plugins.

To reproduce
Steps to reproduce the behavior:

1. Create a fresh virtual environment and activate it. (e.g. python -m venv ./test_venv && source ./test_venv/bin/activate)
2. Use pip to install stactools without the [validate] option. (e.g. pip install stactools or pip install 'stactools[s3]'
3. Try to run stac. (e.g. stac --help)
4. stac throws a ModuleNotFoundError.

Expected behavior
I should be able to run stac --help.

Screenshots and shell session dumps

> ../venv/bin/stac --help            
Traceback (most recent call last):
  File "/home/mgordon/tmp/stactools/../venv/bin/stac", line 5, in <module>
    from stactools.cli.cli import run_cli
  File "/home/mgordon/tmp/venv/lib/python3.12/site-packages/stactools/cli/__init__.py", line 110, in <module>
    registry.load_plugins()
  File "/home/mgordon/tmp/venv/lib/python3.12/site-packages/stactools/cli/registry.py", line 43, in load_plugins
    name: importlib.import_module(name)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/importlib/__init__.py", line 90, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/mgordon/tmp/venv/lib/python3.12/site-packages/stactools/testing/__init__.py", line 4, in <module>
    from .test_data import TestData
  File "/home/mgordon/tmp/venv/lib/python3.12/site-packages/stactools/testing/test_data.py", line 9, in <module>
    import requests
ModuleNotFoundError: No module named 'requests'