Disable escaping "*" on signature

[tk0miya]

On handling autodoc-process-signature event, it's not needed to
escape "*" characters. The escaping is mainly allowed for
highlighting feature of editors. So it's not necessary for
generating contents by autodoc.

In addition, since Sphinx-3.0, the escaping is not recommended by
default (see strip_signature_backslash)

refs: sphinx-doc/sphinx#7439

[larsoner]

Changes LGTM but it would be nice to see the effect before and after. I'd like to see if we can squeak in #258 first, then merge this, if that seems reasonable to you @tk0miya

[tk0miya]

Changes LGTM but it would be nice to see the effect before and after.

Before Sphinx-3.0, backslashes are removed before processing. So this changes nothing.
https://github.com/sphinx-doc/sphinx/blob/2.x/sphinx/directives/__init__.py#L95-L97

Since Sphinx-3.0, backslashes are removed before processing only if strip_signature_backslash is enabled (optional). So backslashes will be shown without this change (or enable strip_signature_backslash on each project). This is one of incompatible changes in our latest release. Anyway, not to escape also works fine for both versions.
https://github.com/sphinx-doc/sphinx/blob/3.x/sphinx/directives/__init__.py#L95-L100

[larsoner]

Thanks @tk0miya