Refactor optional imports

[awaelchli]

Fixes #215

TLDR: Moves optional imports and checks to local imports. This avoids import issues for unrelated features.
See example in #215 why importing optional dependencies at module-level can be problematic.

[gitguardian]

⚠️ GitGuardian has uncovered 4 secrets following the scan of your pull request.

Please consider investigating the findings and remediating the incidents. Failure to do so may lead to compromising the associated services or software components.

🔎 Detected hardcoded secrets in your pull request

GitGuardian id	GitGuardian status	Secret	Commit	Filename
5685611	Triggered	Generic High Entropy Secret	2b0a0ba	tests/streaming/test_resolver.py	View secret
5685611	Triggered	Generic High Entropy Secret	2b0a0ba	tests/streaming/test_resolver.py	View secret
5685611	Triggered	Generic High Entropy Secret	2b0a0ba	tests/streaming/test_resolver.py	View secret
5685611	Triggered	Generic High Entropy Secret	2b0a0ba	tests/streaming/test_resolver.py	View secret

🛠 Guidelines to remediate hardcoded secrets

1. Understand the implications of revoking this secret by investigating where it is used in your code.
2. Replace and store your secret safely. Learn here the best practices.
3. Revoke and rotate this secret.
4. If possible, rewrite git history. Rewriting git history is not a trivial act. You might completely break other contributing developers' workflow and you risk accidentally deleting legitimate data.

To avoid such incidents in the future consider

• following these best practices for managing and storing secrets including API keys and other credentials
• install secret detection on pre-commit to catch secret before it leaves your machine and ease remediation.

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