Cut a release that includes support for keyword-only arguments

[Bazel 2.1 instroduced Starlark changes that allow keyword-only parameters](https://blog.bazel.build/2020/02/06/bazel-2.1.html).

For example:

```bzl
def foo(arg1, arg2, *, kwonly1):
pass
```

But running Stardoc 0.4.0 on `.bzl` files with that new syntax fails with:

```
Stardoc documentation generation failed: Starlark evaluation error
```

Reproduce the failure by running Stardoc 0.4.0 against a .bzl file using the keyword-only example in the issue. Trace the Starlark evaluation failure, then verify that documentation generation succeeds and that a release includes support for this syntax.

Written by the indexing model from the issue text.