Make sure pyproject.toml contains the required properties.

This release includes significant refactoring to modernize the codebase
and improve parser generation performance by a factor of 7 to 15
depending on whether the mypycified version is used.

There are breaking changes in the exported classes:

• Symbol, Token, and Nonterm no longer take parser as the
  argument.

• The Spec class is no longer subclassable. This is mostly due to its
  mypycification, but also because it's supposed to be an opaque piece
  of parser state.

Changelog

• Droped support for Python <= 3.7
  (by @elprans in 4d8c72a)

• Reformat with Black
  (by @elprans in 468ecd0)

• Add thorough type annotations
  (by @elprans in 995c746)

• Massively speedup parser table generation
  (by @elprans in acb3a70)

• Stop passing parser instance to Symbol instances
  (by @elprans in c4ce270)

• Add the ability to specify a callback directly after Spec unpickling
  (by @elprans in bd2e59e)

• Add the ability to build parsing with mypyc
  (by @elprans in 657f3cc)