Shrink ./kmk/ with nox & mpy-cross.

Shrink ./kmk/ with `nox` & `mpy-cross`.

The nice!nano CircuitPython doesn't leave enough storage for an
uncompiled ./kmk/ folder, with main.py and Bluetooth libraries.

This introduces a soft dependancy to kmk on `nox`, as a OS-agnostic
alternative to `make`. Bonus is the use of native python to describe
steps / options / sessions.

noxfile.py

@@ -0,0 +1,56 @@
+import nox
+import shutil
+from pathlib import Path
+
+source_dir = Path('kmk')
+build_dir = Path('build')
+
+
+@nox.session
+def black(session):
+    '''Format python code with `black`.'''
+    session.install('black')
+    session.run('black', source_dir)
+
+
+@nox.session
+def isort(session):
+    session.install('isort')
+    session.run('isort', source_dir)
+
+
+@nox.session
+def flake8(session):
+    session.install('flake8')
+    session.run('flake8', source_dir)
+
+
+@nox.session
+def clean(session):
+    build_dir.mkdir(exist_ok=True)
+    for child in build_dir.iterdir():
+        if child.is_file():
+            child.unlink()
+        else:
+            shutil.rmtree(child)
+
+
+@nox.session
+def compile(session):
+
+    clean(session)
+
+    shutil.copy2('boot.py', 'build/boot.py')
+
+    # Make sure the full folder heirarchy exists
+    for d in source_dir.glob('**/'):
+        if not build_dir.joinpath(d).exists():
+            Path.mkdir(build_dir.joinpath(d))
+
+    # Compile every python file
+    for x in source_dir.glob('**/*.py'):
+        out_path = str(build_dir.joinpath(x).with_suffix('.mpy'))
+        session.run('mpy-cross', f'{x}', '-o', f'{out_path}', external=True)
+
+
+nox.options.sessions = ['black', 'isort']  # Default sessions