Depreciation warning for python 3.8

Hi there,
Nice library. There's a depreciation warning in 3.8.

```
construct/core.py:3: DeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses import struct, io, binascii, itertools, collections, pickle, sys, os, tempfile, hashlib, importlib, imp
```

Thanks, Phil