pw_cpu_exception_cortex_m: Add py.typed file

Adds a py.typed file to the module.

No-Docs-Update-Reason: Added empty file to enable mypy type checking.
Change-Id: I37eb2b8bf1d02da2bfe94a4f29a4c9cef22fd392
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/40940
Reviewed-by: Wyatt Hepler <hepler@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Armando Montanez <amontanez@google.com>
diff --git a/pw_cpu_exception_cortex_m/py/pw_cpu_exception_cortex_m/py.typed b/pw_cpu_exception_cortex_m/py/pw_cpu_exception_cortex_m/py.typed
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/pw_cpu_exception_cortex_m/py/pw_cpu_exception_cortex_m/py.typed