Initial support for PEP 621 metadata with Flit

[amyreese]

This adds a Pep621Reader class that pulls metadata from the [project]
table of pyproject.toml. The FlitReader class uses this as a basis
for reading metadata, and is also updated to not fail if the flit table
is missing.

Tested with checkout of tomli:

(.venv) jreese@mordin ~/workspace/dowsing pep621  » python -m dowsing.pep517 $workspace/tomli/ | jq .get_metadata
{
  "metadata_version": "2.1",
  "name": "tomli",
  "version": "2.0.0",
  "description": "A lil' TOML parser",
  "keywords": [
    "toml"
  ],
  "license": "file: LICENSE",
  "classifiers": [
    "License :: OSI Approved :: MIT License",
    "Operating System :: MacOS",
    "Operating System :: Microsoft :: Windows",
    "Operating System :: POSIX :: Linux",
    "Programming Language :: Python :: 3 :: Only",
    "Programming Language :: Python :: 3.7",
    "Programming Language :: Python :: 3.8",
    "Programming Language :: Python :: 3.9",
    "Programming Language :: Python :: 3.10",
    "Programming Language :: Python :: Implementation :: CPython",
    "Programming Language :: Python :: Implementation :: PyPy",
    "Topic :: Software Development :: Libraries :: Python Modules",
    "Typing :: Typed"
  ],
  "requires_python": ">=3.7",
  "project_urls": {
    "Homepage": "https://github.com/hukkin/tomli",
    "Changelog": "https://github.com/hukkin/tomli/blob/master/CHANGELOG.md"
  },
  "packages": [
    "tests"
  ],
  "packages_dict": {
    "tests": "tests"
  }
}

#28