New image clipping methods

Added

• new round_clip() & elliptic_clip() image clipping methods: link to docs
• CoerciveEnum subclasses have been added: Align & RenderStyle
• documentation on how to embed Matplotlib charts: link to docs
• documentation on how to use fpdf2 with Flask, streamlit, AWS lambdas: link to docs
• documentation on how to store PDFs in a database with SQLAlchemy: link to docs

Modified

• TextMode, XPos & YPos now inherit from CoerciveEnum and hence can simply be passed as string parameters

Fixed

• infinite loop when calling .multi_cell() without enough horizontal space - cf. #389

Removed

• support for .pkl files passed to add_font(). This was deprecated since v2.5.1.
  As a consequence, fpdf2 no more uses the pickle module.