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.

plugins/python-build/test/build/test.bats

@@ -0,0 +1,21 @@
+#!/usr/bin/env bats
+
+load ../test_helper
+export PATH="$BATS_TEST_DIRNAME/../../bin:$PATH"
+
+export PYTHON_BUILD_VERSION="${PYTHON_BUILD_VERSION:-3.8-dev}"
+
+@test "Python build works" {
+  run python-build "$PYTHON_BUILD_VERSION" "$BATS_TMPDIR/dist"
+  assert_success
+
+  [ -e "$BATS_TMPDIR/dist/bin/python" ]
+  run "$BATS_TMPDIR/dist/bin/python" -V
+  assert_success
+  "$BATS_TMPDIR/dist/bin/python" -V >&3 2>&3
+
+  [ -e "$BATS_TMPDIR/dist/bin/pip" ]
+  run "$BATS_TMPDIR/dist/bin/pip" -V
+  assert_success
+  "$BATS_TMPDIR/dist/bin/pip" -V >&3 2>&3
+}