fix(pytest): Correct starlark deps (#637)

The dependencies set in the `deps` are not loaded in
`py_pytest_main.bzl`.

This enables downstream targets to properly depend on
`@aspect_rules_py//py:defs`

Otherwise we're running into an error:

```
ERROR: error loading package '@@rules_python+//docs': Unable to find package for @@[unknown repo 'dev_pip' requested from @@rules_python+]//:requirements.bzl: The repository '@@[unknown repo 'dev_pip' requested from @@rules_python+]' could not be resolved: No repository visible as '@dev_pip' from repository '@@rules_python+'.
ERROR: /private/var/tmp/_bazel_user/7870d530ca6897c426c1d0f00687a4c1/external/aspect_rules_py+/py/private/BUILD.bazel:62:12: error loading package '@@rules_python+//docs': Unable to find package for @@[unknown repo 'dev_pip' requested from @@rules_python+]//:requirements.bzl: The repository '@@[unknown repo 'dev_pip' requested from @@rules_python+]' could not be resolved: No repository visible as '@dev_pip' from repository '@@rules_python+'. and referenced by '@@aspect_rules_py+//py/private:py_pytest_main'
```

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---

### Test plan

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

- Covered by existing test cases
- New test cases added
- Manual testing; please provide instructions so we can reproduce:

```starlark
bzl_library(
    name = "foo",
    srcs = ["foo.bzl"],
    deps = [
       "@aspect_rules_py//py:defs",
   ],
)
```

Author: mgred

py/private/BUILD.bazel

@@ -63,8 +63,7 @@ bzl_library(
     name = "py_pytest_main",
     srcs = ["py_pytest_main.bzl"],
     deps = [
-        "@rules_python//docs:bazel_repo_tools",
-        "@rules_python//python:defs_bzl",
+        ":py_library",
     ],
 )