Update version to 26.3.1 and changelog

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: rtimms

CHANGELOG.md

@@ -1,3 +1,17 @@
+# [Unreleased](https://github.com/pybamm-team/PyBaMM/)
+
+## Features
+
+- Adds EIS support via `EISSimulation` class with a restructure to the `Solution` class to support `EISSolution`. Includes examples and citation, with a ~5x solve improvement over pybamm-eis. ([#5433](https://github.com/pybamm-team/PyBaMM/pull/5433))
+- Refactors `Simulation` class into an inherited class from a `BaseSimulation`. `BaseSimulation` is used for all non-experiment based simulations, with the `Simulation` class adding experiment support. Includes a small code cleanup. ([#5430](https://github.com/pybamm-team/PyBaMM/pull/5430))
+- Improved pchip interpolation performance. ([#5436](https://github.com/pybamm-team/PyBaMM/pull/5436))
+- Refactored CasADi codegen conversion to use OOP dispatch. ([#5436](https://github.com/pybamm-team/PyBaMM/pull/5436))
+- Added a unified experiment-model path that reuses a single built model and solver across compatible experiment steps. ([#5422](https://github.com/pybamm-team/PyBaMM/pull/5422))
+
+## Bug fixes
+
+## Breaking changes
+
 # [v26.3.1](https://github.com/pybamm-team/PyBaMM/tree/v26.3.1) - 2026-04-10
 
 ## Bug fixes

CITATION.cff

@@ -24,6 +24,6 @@ keywords:
   - "expression tree"
   - "python"
   - "symbolic differentiation"
-version: "26.3.0"
+version: "26.3.1"
 repository-code: "https://github.com/pybamm-team/PyBaMM"
 title: "Python Battery Mathematical Modelling (PyBaMM)"