Follow the IOExpander_Base API of M5Unified 0.2.21

[ainyan03]

Problem

M5Unified 0.2.21 changed the IOExpander_Base interface that PY32IOExpander_Class derives from: the mutating methods now return the write status as bool, setPullMode() takes a gpio_pull_t (pull_none / pull_up / pull_down) instead of a bool, and enablePull() was removed from the base class. StackChan-BSP 1.1.0 still declares the old signatures with override, so it no longer compiles against 0.2.21 (m5stack/M5Unified#346):

PY32IOExpander.hpp:26:10: error: conflicting return type specified for 'virtual void m5::PY32IOExpander_Class::setDirection(uint8_t, bool)'
PY32IOExpander.hpp:28:10: error: 'void m5::PY32IOExpander_Class::enablePull(uint8_t, bool)' marked 'override', but does not override
...
error: invalid new-expression of abstract class type 'm5::PY32IOExpander_Class'

Change

• Move the register access into version independent private helpers that return the I2C status and validate the pin (0–13, matching the register layout).
• Select the set of override declarations from M5UNIFIED_VERSION_*, so the library builds against both M5Unified 0.2.20 (old interface) and 0.2.21 and later (new interface).
• Keep the public API of PY32IOExpander_Class source compatible on both versions: setPullMode(pin, bool) and enablePull(pin, bool) remain callable (as plain members on 0.2.21+), and the gpio_pull_t enum is provided locally on the old interface so setPullMode(pin, PY32IOExpander_Class::pull_up) works everywhere. The internal call sites in M5StackChan.cpp use the enum.
• Behaviour on valid pins is unchanged: setHighImpedance(true) still switches the pin to input with both pulls disabled, setPullMode(pin, true) still means pull-up.

Verification

Compiled a sketch that calls M5StackChan.begin() plus the old-style and enum-style pull/drive calls, with PlatformIO (pioarduino 55.03.34, m5stack-cores3, Arduino) against:

• M5Unified 0.2.21 (develop) + M5GFX 0.2.28 — fails on the unmodified library with the errors above, builds with this change
• M5Unified 0.2.20 + M5GFX 0.2.28 — builds before and after

Fixes the build error reported in m5stack/M5Unified#346.

[ainyan03]

CI status of this PR:

• Formatting Check (src): one line of src/M5StackChan.cpp was not clang-formatted (comment alignment after my change). Fixed and force-pushed; the examples job was only cancelled by fail-fast, not failing on its own.
• Lint Check: pre-existing, fails on every run of main since April (LP018: library not in the Library Manager index, LP052: includes= lists directories). Not related to this PR; fixed separately in Fix the Arduino Lint check #9.

The re-run needs a maintainer approval since the PR comes from a fork.