Enable pantsbuild language backends (#5725)

Enable language backends

https://www.pantsbuild.org/docs/initial-configuration\#enable-backends

This does not add any lint/fmt/etc backends, just the language-specific ones.

The experimental.python backend is required for twine support.
Twine is used to publish wheels to pypi.

CHANGELOG.rst

@@ -53,7 +53,7 @@ Added
 
 * Begin introducing `pants <https://www.pantsbuild.org/docs>`_ to improve DX (Developer Experience)
   working on StackStorm, improve our security posture, and improve CI reliability thanks in part
-  to pants' use of PEX lockfiles. This is not a user-facing addition. #5713 #5724 #5726
+  to pants' use of PEX lockfiles. This is not a user-facing addition. #5713 #5724 #5726 #5725
   Contributed by @cognifloyd
 
 Changed

pants.toml

@@ -1,5 +1,14 @@
 [GLOBAL]
 pants_version = "2.13.0rc2"
+backend_packages = [
+  # python
+  "pants.backend.python",
+  "pants.backend.experimental.python", # activates twine `publish` support
+  "pants.backend.python.mixed_interpreter_constraints",
+
+  # shell
+  "pants.backend.shell",
+]
 
 [source]
 # recording each pack individually under root patterns is not great, but resolves these issues: