doc: make doc testable

doc/_readme/llgo_call_py/call_py.go

@@ -0,0 +1,12 @@
+package main
+
+import (
+	"github.com/goplus/llgo/py"
+	"github.com/goplus/llgo/py/math"
+	"github.com/goplus/llgo/py/std"
+)
+
+func main() {
+	x := math.Sqrt(py.Float(2))       // x = sqrt(2)
+	std.Print(py.Str("sqrt(2) ="), x) // print("sqrt(2) =", x)
+}