rust-cpython: fix discrepancy in internal FFI lib version

The dependency to `sys-python3` stayed at version 0.7.1 when `cpython`
got bumped to 0.7.2 for Python 3.12 support. In pratice, this does not
change much because Cargo rules imply that this means ">= 0.7.1, <0.8.0".

Still it did not feel right, either `cpython` enforces a stricter version
and this specification is not needed at all, or it meant that it was still
possible to install the older version of `sys-python3`.

rust/hg-cpython/Cargo.toml

@@ -18,4 +18,4 env_logger = "0.11"
 stable_deref_trait = "1.2.0"
 vcsgraph = "0.2.0"
 logging_timer = "1.1.0"
-python3-sys = "0.7.1"
+python3-sys = "0.7.2"