Update soupsieve to 2.1

[pyup-bot]

This PR updates soupsieve from 2.0.1 to 2.1.

Changelog

2.1.0

- **NEW**: Officially support Python 3.9.
- **NEW**: Drop official support for Python 3.5.
- **NEW**: In order to avoid conflicts with future CSS specification changes, non-standard pseudo classes will now start with the `:-soup-` prefix. As a consequence, `:contains()` will now be known as `:-soup-contains()`, though for a time the deprecated form of `:contains()` will still be allowed with a warning that users should migrate over to `:-soup-contains()`.
- **NEW**: Added new non-standard pseudo class `:-soup-contains-own()` which operates similar to `:-soup-contains()` except that it only looks at text nodes directly associated with the currently scoped element and not its descendants.
- **FIX**: Import `bs4` globally instead of in local functions as it appears there are no adverse affects due to
circular imports as `bs4` does not immediately reference `soupsieve` functions and `soupsieve` does not immediately reference `bs4` functions. This should give a performance boost to functions that had previously included `bs4` locally.

Links

• PyPI: https://pypi.org/project/soupsieve
• Changelog: https://pyup.io/changelogs/soupsieve/
• Repo: https://github.com/facelessuser/soupsieve

Resolves #2551