Adopt PEP 621 Conventions

[moshez]

Fixes #1203

Simple helloworld pyproject.toml after these changes:

[project]
name = "helloworld"
version = "1.2.3"
authors = [{name = "Jane Developer", email = "[email protected]"}]
dependencies = ["numpy"]
description = "very silly"

[project.urls]
Homepage = "https://example.com/helloworld"

[project.optional-dependencies]
test = ["pytest"]

[project.license]
text = "BSD....."

[tool.briefcase]
project_name = "Hello World"
bundle = "com.example"

[tool.briefcase.app.helloworld]
formal_name = "Hello World"
long_description = """More details about the app should go here.
"""
icon = "src/helloworld/resources/helloworld"
sources = [
    "src/helloworld",
]
test_sources = [
    "tests",
]

[tool.briefcase.app.helloworld.macOS]
requires = [
    "toga-cocoa~=0.3.1",
    "std-nslog~=1.0.0"
]

PR Checklist:

• All new features have been tested
• All new features have been documented
• have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

I've tweaked the docs and added some more tests of the merging process, but otherwise this looks great - thanks for the PR!