build: Add packaging build system

[matthewfeickert]

• Use src/ directory layout.
• Add missing metadata, such as requires-python.
• Add packaging support using hatch build-system.

Add packaging support. Enables

$ uvx --from build pyproject-build .
* Creating isolated environment: venv+pip...
* Installing packages in isolated environment:
  - hatchling>=1.13.0
* Getting build dependencies for sdist...
* Building sdist...
* Building wheel from sdist
* Creating isolated environment: venv+pip...
* Installing packages in isolated environment:
  - hatchling>=1.13.0
* Getting build dependencies for wheel...
* Building wheel...
Successfully built func_adl_type_generator-0.1.0.tar.gz and func_adl_type_generator-0.1.0-py3-none-any.whl
$ uvx twine check --strict dist/*
Checking dist/func_adl_type_generator-0.1.0-py3-none-any.whl: PASSED
Checking dist/func_adl_type_generator-0.1.0.tar.gz: PASSED
$ python -m tarfile --list dist/*.tar.gz
func_adl_type_generator-0.1.0/src/func_adl_type_generator/atlas_release_type_builder.py 
func_adl_type_generator-0.1.0/.gitignore 
func_adl_type_generator-0.1.0/LICENSE 
func_adl_type_generator-0.1.0/README.md 
func_adl_type_generator-0.1.0/pyproject.toml 
func_adl_type_generator-0.1.0/PKG-INFO 
$ python -m zipfile --list dist/*.whl
File Name                                             Modified             Size
func_adl_type_generator/atlas_release_type_builder.py 2020-02-02 00:00:00        12020
func_adl_type_generator-0.1.0.dist-info/METADATA 2020-02-02 00:00:00         2300
func_adl_type_generator-0.1.0.dist-info/WHEEL  2020-02-02 00:00:00           87
func_adl_type_generator-0.1.0.dist-info/entry_points.txt 2020-02-02 00:00:00          103
func_adl_type_generator-0.1.0.dist-info/licenses/LICENSE 2020-02-02 00:00:00        11357
func_adl_type_generator-0.1.0.dist-info/RECORD 2020-02-02 00:00:00          591

[matthewfeickert]

@gordonwatts This doesn't address your question from the other day fully, but this allows for things to be packaged correctly, which allows for PR #4, which will start to simplify things.

This is ready for review.

[matthewfeickert]

I'm setting this here as it is really important to set some Python version. If this needs to get bumped down to Python 3.7 that's fine, but it shouldn't be absent.