Skip dir() names that raise AttributeError when gathering visitors

[Sanjays2402]

Closes #1458

Summary

_gather_matchers and the two _gather_constructed_*_funcs helpers iterate over dir(obj) and getattr every name. A name reported by dir() isn't guaranteed to be retrievable — zope.interface installs a __provides__ descriptor that raises AttributeError on access — so merely importing Twisted made every MatcherDecoratableVisitor/Transformer constructor crash with AttributeError: __provides__. Such names can't be visit/leave methods, so they're now fetched through a _safe_getattr helper and skipped.

Test Plan

New test_inaccessible_attribute_is_skipped in libcst/matchers/tests/test_visitors.py uses a descriptor that raises AttributeError on access; it fails with the reported AttributeError: __provides__ without the fix and passes with it. libcst/matchers/tests/ + test_add_imports.py = 180 passed; black and flake8 clean on the changed files.

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