Apply <> for delimiting instance params in str function of Vocab class

[policyglot]

Fixes #2890

Please ignore the name of the branch- I was working on another idea when I noticed the open issue above.

Replaced double quotes with <>. This fix does not change the behavior of the 'str' function within the Vocab class of keyedvectors.py. It is only intended to address the change in conventions for delimiting instance parameters.

[gojomo]

Note that the original Vocab class no longer exists, except as a temporary renamed CompatVocab shim retained for backward compatibility when deserializing older models. (Still, its instances are thrown away as soon as the loaded models are converted to current format - so while this formatting fix could be applied, it's a largely moot issue.)

If you're relying on the old Vocab class behavior at all - especially the way arbitrary extra attributes could be added – be sure to take a look at the big changes in the current develop branch, building towards gensim-4.0.0, where you'll need to use a new facility (get_vecattr(), set_vecattr()) to hang new values on word-vectors. (If your extra attributes are the same type and applied to all items, the new approach may offer a speed/size benefit.)

[piskvorky]

PR doesn't merge currently, there are conflicts. Can you fix that?
But like @gojomo says, these conflicts may be fundamental – please make sure to start off any branches off the current develop.

[piskvorky]

Closing – please let us know if you see any such str formatting issues in the latest Gensim.