fix: allow test venv to be run (#348)

Allows the tests venv target to be run, creating the virtual env for the
test target.

See
#15 (comment)

---

### Changes are visible to end-users: yes

- Searched for relevant documentation and updated as needed: yes
- Breaking change (forces users to change their own code or config): no
- Suggested release notes appear below: yes

It's now possible to run the venv target produced by a `py_test`

### Test plan

<!-- Delete any which do not apply -->

- Manual testing; please provide instructions so we can reproduce:
`bazel run :some_test.venv` works now in a fresh repo

Author: alexeagle

py/defs.bzl

@@ -46,6 +46,7 @@ def _py_binary_or_test(name, rule, srcs, main, imports, deps = [], resolutions =
         imports = imports,
         resolutions = resolutions,
         tags = ["manual"],
+        testonly = kwargs.get("testonly", False),
     )
 
 def py_binary(name, srcs = [], main = None, imports = ["."], **kwargs):