fix compiled-objects-have-debug-symbols on macOS

[jameslamb]

Running this project's unit tests on my mac today (macOS 12.2.1, intel chip), I found 2 test failures that don't show up in CI:

E       AssertionError: ==================== running pydistcheck ====================
E         
E         checking '/Users/jlamb/repos/pydistcheck/tests/data/baseballmetrics-0.1.0-py3-none-macosx_10_15_x86_64.macosx_11_6_x86_64.macosx_12_5_x86_64.whl'
E         ------------ check results -----------
E         1. [compiled-objects-have-debug-symbols] Found compiled object containing debug symbols. For details, extract the distribution contents and run 'nm --debug-syms "lib/lib_baseballmetrics.dylib"'.

It seems that the nm distributed with Xcode does not support argument --debug-syms (docs), while GNU nm (docs) and llvm-nm (docs) do.

All 3 support -a to accomplish the same thing.

This proposes using nm -a instead of nm --debug-syms.

Benefits of this change

Removes a source of false positives for the compiled-objects-have-debug-symbols check.