Review code to take into account the changes from MkDocs 1.4.0

[prcr]

Go through the breaking changes from the MkDocs 1.4.0 changelog and make any necessary changes to the code to ensure that it is compatible with MkDocs 1.4.0.

Supersedes #193.

[codeautopilot]

Based on the file-level summary, it appears that no changes are needed in the project's setup.py and plugin.py files to ensure compatibility with MkDocs 1.4.0. Therefore, you can continue to use these files as is without any modifications.

Changes on setup.py

No changes are needed in this file to ensure compatibility with MkDocs 1.4.0.

Changes on mkdocs_meta_descriptions_plugin/plugin.py

No changes are needed in this file based on the MkDocs 1.4.0 changelog.

[prcr]

Consider applying the following:

• Replace File.src_path to not deal with backslashes
• Type annotations for event handler methods
• Rework ConfigOption schemas as class-based