Switch build backend to Hatchling (#3233)

Switch build backend to Hatchling (#3233)

This implements PEP 621, obviating the need for `setup.py`, `setup.cfg`,
and `MANIFEST.in`. The build backend Hatchling (of which I am a
maintainer in the PyPA) is now used as that is the default in the
official Python packaging tutorial. Hatchling is available on all the
major distribution channels such as Debian, Fedora, and many more.

## Python support

The earliest supported Python 3 version of Hatchling is 3.7, therefore
I've also set that as the minimum here. Python 3.6 is EOL and other
build backends like flit-core and setuptools also dropped support.
Python 3.6 accounted for 3-4% of downloads in the last month.

## Plugins

Configuration is now completely static with the help of 3 plugins:

### Readme

hynek's hatch-fancy-pypi-readme allows for the dynamic construction of
the readme which was previously coded up in `setup.py`. Now it's simply:

```toml
[tool.hatch.metadata.hooks.fancy-pypi-readme]
content-type = "text/markdown"
fragments = [
  { path = "README.md" },
  { path = "CHANGES.md" },
]
```

### Versioning

hatch-vcs is currently just a wrapper around setuptools-scm (which
despite the legacy naming is actually now decoupled from setuptools):

```toml
[tool.hatch.version]
source = "vcs"

[tool.hatch.build.hooks.vcs]
version-file = "src/_black_version.py"
template = '''
version = "{version}"
'''
```

### mypyc

hatch-mypyc offers many benefits over the existing approach:

- No need to manually select files for inclusion
- Avoids the need for the current CI workaround for https://github.com/mypyc/mypyc/issues/946
- Intermediate artifacts (like `build/`) from setuptools and mypyc
  itself no longer clutter the project directory
- Runtime dependencies required at build time no longer need to be
  manually redeclared as this is a built-in option of Hatchling

Co-authored-by: Richard Si <63936253+ichard26@users.noreply.github.com>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>