Web Assets 2.1.0

Vendor webassets to future-proof the plugin. As a drive-by we have dropped
Python versions below 3.9, which are not supported by the full set of this
plugin's' dependencies.

Web Assets 2.0.0

• Consolidate assets and pelican-webassets plugins into a single project
• Support plugin auto-registration via new namespace plugin format on Pelican 4.5+
• Deprecate ASSET_* settings in favor of WEBASSETS_*
• Fix bug where an empty THEME_STATIC_DIR setting would create malformed URLs