Pytest-localstack

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- Update to docker >= v6 to address deprecation warnings.

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- Get the default Localstack hostname from the `DOCKER_HOST` env var, if set.
- Protect starting/stopping the Localstack container with a lock.

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- Drop support for Python < 3.6.
- Switch to using the new single port for localstack >= 0.11.0.
- Fix bug in STS client endpoint patching.
- Update patching to be compatible with newer botocore versions.

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- Raise TimeoutErrors when services fail to start from any causing exception.
- Use more botocore client-based service checks.

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- Add EC2 service.
- Add IAM service.
- Add Secret Manager service.
- Add Step Functions service.
- Add STS service.

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- Track/restore original boto3.DEFAULT_SESSION during patching.