Soupsieve

- **FIX**: Fix issue with undefined namespaces.

- **NEW**: Add support for `:scope`.
- **NEW**: `:user-invalid`, `:playing`, `:paused`, and `:local-link` will not cause a failure, but all will match nothing as their use cases are not possible in an environment outside a web browser.
- **FIX**: Fix `[attr~=value]` handling of whitespace. According to the spec, if the value contains whitespace, or is an empty string, it should not match anything.
- **FIX**: Precompile internal patterns for pseudo-classes to prevent having to parse them again.

- **FIX**: More descriptive exceptions. Exceptions will also now mention position in the pattern that is problematic.
- **FIX**: `filter` ignores `NavigableString` objects in normal iterables and `Tag` iterables. Basically, it filters all Beautiful Soup document parts regardless of iterable type where as it used to only filter out a `NavigableString` in a `Tag` object. This is viewed as fixing an inconsistency.
- **FIX**: `DEBUG` flag has been added to help with debugging CSS selector parsing. This is mainly for development.
- **FIX**: If forced to search for language in `meta` tag, and no language is found, cache that there is no language in the `meta` tag to prevent searching again during the current select.
- **FIX**: If a non `BeautifulSoup`/`Tag` object is given to the API to compare against, raise a `TypeError`.

- **NEW**: Add Python 2.7 support.
- **NEW**: Remove old pre 1.0 deprecations.

- **NEW**: Adds support for `[attr!=value]` which is equivalent to `:not([attr=value])`.
- **NEW**: Add support for `:active`, `:focus`, `:hover`, `:visited`, `:target`, `:focus-within`, `:focus-visible`, `:target-within`, `:current()`/`:current`, `:past`, and `:future`, but they will never match as these states don't exist in the Soup Sieve environment.
- **NEW**: Add support for `:checked`, `:enabled`, `:disabled`, `:required`, `:optional`, `:default`, and `:placeholder-shown` which will only match in HTML documents as these concepts are not defined in XML.
- **NEW**: Add support for `:link` and `:any-link`, both of which will target all `<a>`, `<area>`, and `<link>` elements with an `href` attribute as all links will be treated as unvisited in Soup Sieve.
- **NEW**: Add support for `:lang()` (CSS4) which works in XML and HTML.
- **NEW**: Users must install Beautiful Soup themselves. This requirement is removed in the hopes that Beautiful Soup may use this in the future.
- **FIX**: Attributes in the form `prefix:attr` can be matched with the form `[prefix\:attr]` without specifying a namespaces if desired.
- **FIX**: Fix exception when `[type]` is used (with no value).

- **FIX**: Use proper CSS identifier patterns for tag names, classes, ids, etc. Things like `3` or `-3` should not match and should require `\33` or `-\33`.
- **FIX**: Do not raise `NotImplementedError` for supported pseudo classes/elements with bad syntax, instead raise `SyntaxError`.