fix: update blockscout verifier and improvement

[s3bc40]

Related issues:

• Smart contract verification process seems to be broken on sepolia testnets #403

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What I did

Fix compatibility Blockscout APIs by introducing a unified session management utility, and updating the Blockscout verification logic to remove reliance on API keys.

How I did it

Updates to API Compatibility:

• Blockscout Enhancements: Removed the api_key attribute from the Blockscout class and updated verification logic to check contract existence and verification status before proceeding. Introduced retries for contract creation using the session utility.

Session Management Improvements:

• Unified Session Utility: Added get_session in boa/util/cached_session.py to abstract session creation. This utility supports both requests_cache and fallback to regular requests.Session, enabling consistent session handling across the codebase.
• Session Integration: Replaced direct session instantiation with the new get_session utility in boa/explorer.py and boa/verifiers.py.

Test Case Updates:

• Blockscout Tests: Updated integration tests to reflect the removal of API keys for Blockscout.

How to verify it

Run the following test:

pytest tests/integration/network/sepolia/test_sepolia_env.py -vvv -k test_verify

Description for the changelog

• Removed API key dependency
• Streamlined verification logic
• Improved contract existence/status checks.
• Unified session utility to standardize and simplify HTTP session management across the codebase.

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