Switch build backend to hatchling (#268)

This leads to the following improvements:
* Drops all the test files from the wheel
* Add `.gitignore`, remove `CODE_OF_CONDUCT` from sdist

Author: hoodmane

MANIFEST.in

@@ -1,6 +1,6 @@
 [build-system]
-requires = ["setuptools>=61.0", "wheel"]
-build-backend = "setuptools.build_meta"
+requires = ["hatchling"]
+build-backend = "hatchling.build"
 
 [project]
 name = "sphinx-js"
@@ -45,6 +45,19 @@ classifiers = [
 [project.urls]
 Homepage = "https://github.com/pyodide/sphinx-js"
 
+[tool.hatch.build.targets.sdist]
+include = [
+    "/sphinx_js",
+    "/tests",
+    "/LICENSE",
+    "/requirements_dev.txt",
+    "/noxfile.py",
+    "/README.rst",
+]
+
+[tool.hatch.build.targets.wheel]
+packages = ["sphinx_js"]
+
 [tool.mypy]
 python_version = "3.10"
 show_error_codes = true