Introduce immutable RNG state for correct comparisons and pickling

This MR refactors the frontend RNG architecture of !474 (merged) to use immutable RNG state objects.

An immutable state object must be attached to each RNG invocation in order to correctly compare equivalent invocations and allow them to be pickled. The RNG objects themselves now serve as factories for their invocations and associated series of state objects.

• Add RngState class for immutable RNG state objects
• Turn RngBase and subclass Philox into factories for their state
• Remove RngFunc and replace it by an introspection-based typechecking mechanism (inheritance from RngFunc is not preserved by pickle)
• Adapt backend parsing
• Add test cases for RNG comparion and pickling
• Bump SymPy dependency to ~=1.14 (RNG functions need https://github.com/sympy/sympy/pull/26881)
• Fix SymPy-related spurious mypy errors

changed milestone to %Release 2.0

assigned to @da15siwa

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• df4c915a - fix SymPy dependency version

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marked this merge request as ready

requested review from @kuron

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• 898ab904 - fix doc comments

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marked this merge request as draft

removed review request for @kuron

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• b9d86e7c - extend pickle test to check RngFunc types

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• da6cb8a7 - Replace `RngFunc` by a runtime checking protocol

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marked this merge request as ready

requested review from @kuron

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mentioned in merge request lbmpy!172 (merged)

mentioned in issue #130

Looks good, just some minor issues with two comments.

approved this merge request

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• ffde2c83 - fix doc comments

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enabled an automatic merge when all merge checks for ffde2c83 pass

mentioned in commit 5c4f9d2c

merged