Use the correct cssselect library name in docblocks.

The original python library is called cssselect, not cssselector.
Update the note about origins of the CssSelector component.

The last rewrite is based on the cssselect which is no longer part of lxml, but was extracted as an independent library.
minor #13132 [CssSelector] Use the correct cssselect library name in docblocks. (jakzal)

This PR was merged into the 2.3 branch.

Discussion
----------

[CssSelector] Use the correct cssselect library name in docblocks.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | #11894
| License       | MIT
| Doc PR        | -

As suggested in #11894, the original python library is called [cssselect](https://github.com/SimonSapin/cssselect), not cssselector.

I also updated the README, as the cssselect was extracted as an independent library and it's no longer part of lxml.

Commits
-------

55f0d9e Update the note about origins of the CssSelector component.
1997b12 Use the correct cssselect library name in docblocks.