Fix exception swallowing by Trait attribute access

[mdickinson]

When doing obj.attr_name on a HasTraits object obj, if attr_name doesn't appear in either the class or instance dictionaries, Traits next tries to look it up as a regular Python attribute (via the PyObject_GenericGetAttr C-API function). If that lookup fails with an exception of any kind, the exception is swallowed and Traits then moves on to try the general prefix-trait machinery. In effect, there's a bare except here, but at C level.

This PR modifies the behaviour to replace the bare except with an except AttributeError, so that exceptions other than AttributeError will no longer be swallowed.

The fix itself is easy. Writing a regression test that worked was ... more interesting.

This fixes one case of #946.

[corranwebster]

LGTM.

I assume we have a text case somewhere where AttributeError is correctly raised by the regular access machinery and processing carries on.

[mdickinson]

I assume we have a text case somewhere where AttributeError is correctly raised by the regular access machinery and processing carries on.

I'll double check. But I'd be surprised if we didn't: this should be a very common case.

[mdickinson]

I'll double check. But I'd be surprised if we didn't: this should be a very common case.

Hmm. Lots of occurrences (I count 35 cases from running the test suite), but nothing that's directly exercising this case. I'll add another test.

[codecov-io]

Codecov Report

Merging #959 into master will increase coverage by 0.10%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master     #959      +/-   ##
==========================================
+ Coverage   73.05%   73.16%   +0.10%     
==========================================
  Files          51       51              
  Lines        6514     6507       -7     
  Branches     1309     1308       -1     
==========================================
+ Hits         4759     4761       +2     
+ Misses       1363     1352      -11     
- Partials      392      394       +2     

Impacted Files	Coverage Δ
traits/has_traits.py	71.24% <ø> (ø)
traits/trait_handlers.py	61.11% <ø> (ø)
traits/trait_types.py	71.35% <93.33%> (-0.30%)	⬇️
traits/editor_factories.py	79.59% <0.00%> (-9.19%)	⬇️
traits/traits.py	76.92% <0.00%> (+4.04%)	⬆️
traits/etsconfig/etsconfig.py	63.46% <0.00%> (+6.41%)	⬆️

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[corranwebster]

Additional tests look good 👍