lcmgen python: remove py 2 cStringIO compatibility

Python 2 support has been dropped so the `try ... execpt` of the py 2
module can be removed.

`import cStringIO.StringIO as BytesIO` was confusing pylance with
`Module is not callable`. Therefore it couldn't determain that `encode`
returned `bytes`.

lcmgen/emit_python.c

@@ -834,10 +834,7 @@ emit_package (lcmgen_t *lcm, _package_contents_t *pc)
  "DO NOT MODIFY BY HAND!!!!\n"
  "\"\"\"\n"
  "\n"
- "try:\n"
- " import cStringIO.StringIO as BytesIO\n"
- "except ImportError:\n"
- " from io import BytesIO\n"
+ "from io import BytesIO\n"
  "import struct\n\n");
 
  // enums always encoded as int32
@@ -919,10 +916,7 @@ emit_package (lcmgen_t *lcm, _package_contents_t *pc)
  "DO NOT MODIFY BY HAND!!!!\n"
  "\"\"\"\n"
  "\n"
- "try:\n"
- " import cStringIO.StringIO as BytesIO\n"
- "except ImportError:\n"
- " from io import BytesIO\n"
+ "from io import BytesIO\n"
  "import struct\n\n");
 
  emit_python_dependencies(lcm, f, ls);