Migrate to flint build backend

I've accidentally noticed that my .vscode/settings.json got included
into source distribution, even though it's ignored by git
as configured in my global config. This came as a surprise,
but appears to be a design decision (see pypa/hatch#304).

Switch to flit which by default only includes the package contents,
readme and license.

pyproject.toml

@@ -6,7 +6,7 @@ authors = [
 ]
 description = "Predicate-based equivalence testing"
 readme = "README.rst"
-license = "MIT"
+license = {file = "LICENSE"}
 requires-python = ">=3.8"
 classifiers = [
     "Programming Language :: Python :: 3",
@@ -25,14 +25,8 @@ dev = [
 ]
 
 [build-system]
-requires = ["hatchling"]
-build-backend = "hatchling.build"
-
-[tool.hatch.build]
-exclude = [
-    "/.github",
-    "/test",
-]
+requires = ["flit_core >=3.4,<4"]
+build-backend = "flit_core.buildapi"
 
 [tool.pytest.ini_options]
 addopts = "--doctest-modules"