custom StacIO init needs to call parent init

[tylere]

Related Issue(s):

none

Description: This fixes tutorial code that accesses S3, which probably broke with commit https://github.com/stac-utils/pystac/commit/01f71b15e91762557aa1d2c9374458c5dcef.

PR Checklist:

• [x] pre-commit hooks pass locally
• [x] Tests pass (run scripts/test)
• [x] Documentation has been updated to reflect changes, if applicable
• [x] This PR maintains or improves overall codebase code coverage.
• [x] Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (8004c10) 91.18% compared to head (9c5bc59) 91.18%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1308 +/- ## ======================================= Coverage 91.18% 91.18% ======================================= Files 51 51 Lines 7023 7023 Branches 1005 1005 ======================================= Hits 6404 6404 Misses 444 444 Partials 175 175 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.