Add CI builds that actually build Python.

For now this just builds the latest CPython, but it can be
extended to include other versions.

Skip with `[skip build]` in commit message.

Makefile

@@ -1,4 +1,4 @@
-.PHONY: test
+.PHONY: test test-build
 
 # Do not pass in user flags to build tests.
 unexport PYTHON_CFLAGS
@@ -8,5 +8,8 @@ test: bats
 	PATH="./bats/bin:$$PATH" test/run
 	cd plugins/python-build && $(PWD)/bats/bin/bats $${CI:+--tap} test
 
+test-build: bats
+	cd plugins/python-build && $(PWD)/bats/bin/bats $${CI:+--tap} test/build
+
 bats:
-	git clone --depth 1 https://github.com/sstephenson/bats.git
+	git clone --depth 1 https://github.com/bats-core/bats-core.git bats