Avoid imp import on Python 3

[Mirochill]

Fixes #719.

Summary:

• avoid importing the removed imp module at tdw.flatbuffers.compat import time on Python 3
• use importlib.util.find_spec(numpy) for the Python 3 optional-NumPy check
• keep the existing imp.find_module() path only for Python 2 compatibility

This does not change the advertised Python classifiers; it only removes the immediate import-time blocker reported in the issue.

Validation:

• git diff --check HEAD~1..HEAD
• Static search confirmed remaining imp references are under the PY2 branch, and the Python 3 path uses find_spec.
• Not run locally.