Unify fork changes from systemallica/python-redsys

[systemallica]

Context

This PR prepares the merge between the systemallica/python-redsys fork and the original ddiazpinto/python-redsys repository.

The goal is to bring back the changes that have accumulated in the fork over time, grouped into descriptive commits so they can be reviewed and merged into the original project more easily.

Included changes

• Replaces pycrypto with pycryptodome, keeping a conservative migration from the historical Crypto API.
• Migrates project packaging and tooling to uv, pyproject.toml, ruff, and pre-commit.
• Modernizes Redsys request/response handling and consolidates internal constants.
• Adds support and tests for changes introduced in the fork, including BIZUM, 12-character order IDs, and additional response parameters.
• Adds CI and release workflows.
• Refreshes documentation, changelog, and license metadata.

Validation

• uv run pytest
• Local result: 13 tests passed.

Notes

The branch is organized so each commit represents a reasonably self-contained review unit. The final tree matches the current state of the systemallica/python-redsys fork that is intended to be merged back into the original repository.