Use importlib instead of imp module on python 3.4+

Use importlib instead of imp module on python 3.4+

Python 3.4 has deprecated the imp module in favour of importlib. This
patch avoids the DeprecationWarning. This warning is visible to users
whose libpython.so has been compiled with --with-pydebug.

Considering that even python 3.5 has reached end of life, would it be
better to just use importlib and drop support for python 3.0 to 3.3?

2021-02-28  Boris Staletic  <boris.staletic@gmail.com>

	* gdb/python/lib/gdb/__init__.py: Use importlib on python 3.4+
	to avoid deprecation warnings.