Make exceptions hashable (but not comparable)

[stuarteberg]

This PR addresses #477.

Currently, ValidationError and SchemaError implement __eq__, but don't implement __hash__, which makes then unhashable. That prevents them from being used in certain scenarios, notably in a test suite that uses pytest [Edit: It fails under python 3.6, but not 3.7].

The easiest way to make them hashable is to simply remove the __eq__ implementation. I doubt many users of jsonschema rely on the fact that those errors are comparable.

The only problem is jsonschema's own test suite, which does rely on ValidationError.__eq__(). But that's easy to change, as I've done in this PR. Thoughts?

[codecov]

Codecov Report

Merging #502 into master will increase coverage by 0.21%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #502      +/-   ##
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+ Coverage   94.89%   95.1%   +0.21%     
=========================================
  Files          19      19              
  Lines        2310    2309       -1     
  Branches      302     300       -2     
=========================================
+ Hits         2192    2196       +4     
+ Misses        104     100       -4     
+ Partials       14      13       -1

[Julian]

So, I'd actually like to move in the reverse direction longer term (where users can rely on comparisons, which is what they should do, a large reason why I added this originally was that there were many internal tests that were having to make many assertions instead of just one single comparison).

But! In the short term, I think this does make sense until we do the deprecations and can therefore roll this out more sanely.

So overall, thanks :), looks reasonable.