Test for zstd in Python 3.14+

[tianon]

• related to No module named '_zstd' python#1085

$ ./test/tests/python-imports/run.sh python:3.14-slim
Traceback (most recent call last):
  File "/tmp/test-dir/python-imports/./container.py", line 29, in <module>
    from compression import zstd
  File "/usr/local/lib/python3.14/compression/zstd/__init__.py", line 29, in <module>
    import _zstd
ModuleNotFoundError: No module named '_zstd'