[PyYAML] Improve dump/serialize types (#10196)

* Add overloads to return `str` or `bytes`, instead of `Any`.
* Mark some arguments as keyword-only.
* Add missing default values.

stubs/PyYAML/@tests/stubtest_allowlist.txt

@@ -1,2 +1,8 @@
 # yaml._yaml is for backwards compatibility so none of it matters anyway
 yaml._yaml.__test__
+
+# Some arguments to these functions are technically positional or keyword
+# arguments at runtime, but according to the documentation and other,
+# similar functions, it's safer to treat them as keyword-only arguments.
+yaml.dump_all
+yaml.serialize_all