[MAINTENANCE] Make scipy an optional extra and drop redundant cryptography pin

[dataders]

Summary

• Moves scipy (82MB) out of the default install and into a new great_expectations[scipy] extra. It's only used by three files for scipy.stats (KL-divergence and KS-test expectations). A lazy-import wrapper in great_expectations/compatibility/scipy.py (same pattern as the existing compatibility/pyspark.py/compatibility/snowflake.py) gives a clear error pointing at the scipy extra if those specific expectations are used without it installed.
• Drops cryptography (13MB) from requirements.txt entirely. It was already function-local imported in sqlalchemy_execution_engine.py for Snowflake key-pair auth (no eager cost), and snowflake-connector-python already brings it in transitively for anyone on the snowflake extra — the top-level pin was pure redundant weight.
• Net effect: ~95MB smaller default install, no functional change for existing users of either dependency.

Test plan

• great_expectations and all three scipy-dependent modules (expect_column_kl_divergence_to_be_less_than.py, column_bootstrapped_ks_test_p_value.py, column_parameterized_distribution_ks_test_p_value.py) import cleanly with scipy simulated as absent (verified via sys.modules/builtins.__import__ patching)
• Existing KL-divergence integration test suite passes with scipy installed (tests/integration/data_sources_and_expectations/expectations/test_expect_column_kl_divergence_to_be_less_than.py, 9/9)
• Full tests/expectations, tests/execution_engine, tests/datasource/fluent unit sweep: 2887 passed, 0 failed
• great_expectations[scipy] extra confirmed generated correctly via setup.py::get_extras_require()

🤖 Generated with Claude Code

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[github-actions]

Thanks for the pull request, @dataders! 👋

This change touches files that usually mean new data source or execution engine support:

• great_expectations/compatibility/scipy.py — a new compatibility module (a new optional third-party dependency)
• reqs/requirements-dev-scipy.txt — a new backend requirements file

Changes like that need an RFC agreed before implementation, so the design discussion happens before you invest in code. Sorry if this arrives after the fact — the check is here so the next contributor finds out at the right moment.

To resolve this check, add one of these lines to the pull-request description:

• RFC: <link to the accepted discussion> — if an RFC exists or you open one now
• No RFC needed: <reason> — if this isn't actually new backend support (for example, a bug fix that happens to touch these paths)

Either answer satisfies the check. A maintainer will pick it up from there.

[github-actions]

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