Move dependencies to pyproject.toml

[jtraglia]

This PR moves dependency definitions to a new pyproject.toml file.

This has the following benefits:

• We can remove the "hacky" installPackage function from setup.py:

consensus-specs/setup.py

Lines 49 to 65 in b73f3bd

	# NOTE: have to programmatically include third-party dependencies in `setup.py`.
	def installPackage(package: str):
	subprocess.check_call([sys.executable, '-m', 'pip', 'install', package])
	RUAMEL_YAML_VERSION = "ruamel.yaml==0.17.21"
	try:
	import ruamel.yaml
	except ImportError:
	installPackage(RUAMEL_YAML_VERSION)
	from ruamel.yaml import YAML
	MARKO_VERSION = "marko==1.0.2"
	try:
	import marko
	except ImportError:
	installPackage(MARKO_VERSION)

• All dependencies are located in a single file, which makes it easier to manage.
• No longer need requirements_preinstallation, as these are defined in [build-system] now.
• All of the imports in setup.py can now be moved to the top of the file.
  • I've also ordered these alphabetically & broken them up into sections.

[cheyenne-bot]

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