WooCommerce One Page Checkout 2.9.1

* Tweak: Tooltip display in admin screen of category search.
* Tweak: WC 3.8 compatibility.

* New: Introduce helper to find if it's a frontend request
* New: Bumps WC tested up to version.
* Tweak: Updates notice copy to reflect minimum WC version
* Tweak: Update minimum required WC version
* Tweak: Removes WC 2.5 pre code
* Tweak: Remove duplicate jQuery selectors
* Fix: Hides shipping fields for virtual products
* Fix: Avoid duplicated notices when variables product is not selected
* Fix: Prevent errors while generating page via Gutenberg editor
* Fix: Removes use of deprecated jQuery function size()

* New: Add documentation to the README file for `category_ids` shortcode property. PR #338
* New: Allow disabling the auto-scroll behavior when cart items are added/removed/changed. PR #339
* New: Allow passing custom data to Ajax calls on easy pricing table templates. PR #326
* New: Include category IDs in the shortcode generator (requires WC 3.2+). PR #342
* Fix: Compatibility with Name Your Price hook changes. PR #324
* Fix: [WC 3.2] Ensure `no-index` meta is not displayed on OPC pages. PR #336
* Fix: Do not use `__FILE__` PHP constant for nonce action names. PR #325
* Fix: Only display shipping fields on single product pages when a child product requires shipping. PR #335
* Fix: Include simple children of grouped products on OPC pages. PR #330