move python demos to _pydemo

_pydemo/hellopy/hello.go

@@ -0,0 +1,13 @@
+package main
+
+import (
+	"github.com/goplus/llgo/c"
+	"github.com/goplus/llgo/py"
+)
+
+func main() {
+	py.Initialize()
+	py.SetProgramName(*c.Argv)
+	py.RunSimpleString(c.Str(`print('Hello, World!')`))
+	py.Finalize()
+}