Constrain backports-asyncio-runner in runtime.txt

Per Pavol's suggestion, this is a better approach, and it lets us omit
the `--unsafe-package` flag from run-pip-compile.sh.

Author: mhucka

dev_tools/requirements/deps/runtime.txt

@@ -13,3 +13,9 @@ sympy
 # Transitive dependency. Versions 1.3.3+ require Python 311. Constraining it
 # lets us produce a single set of requirements files for Python 3.10 - 3.12.
 contourpy<1.3.3
+
+# Another transitive dependency. We don't care about the version of this
+# backport because it's only needed for a pytest extension. However, because we
+# run pip-compile in Python 3.10 for maximum compatibility, we need to add an
+# explicit constraint to keep this package out of the requirements files.
+backports-asyncio-runner ; python_full_version < '3.11'