remove unused import
remove unused import

The PR at https://github.com/numba/numba/pull/3646 removed use of the
`traceback` module but didn't remove the import. Hence the `Linux
py35_np111_cov` build on Azure that does a `flake8` check reported the
following error:

```
numba/typeinfer.py:22:1: F401 'traceback' imported but unused
```

This commit fixes that.

cpcloud/numba,sklam/numba,stuartarchibald/numba,stonebig/numba,gmarkall/numba,cpcloud/numba,IntelLabs/numba,IntelLabs/numba,stonebig/numba,seibert/numba,gmarkall/numba,cpcloud/numba,numba/numba,sklam/numba,seibert/numba,cpcloud/numba,sklam/numba,numba/numba,jriehl/numba,jriehl/numba,IntelLabs/numba,stuartarchibald/numba,stuartarchibald/numba,seibert/numba,numba/numba,stuartarchibald/numba,IntelLabs/numba,jriehl/numba,stuartarchibald/numba,stonebig/numba,gmarkall/numba,stonebig/numba,jriehl/numba,jriehl/numba,numba/numba,numba/numba,IntelLabs/numba,sklam/numba,gmarkall/numba,gmarkall/numba,sklam/numba,stonebig/numba,seibert/numba,cpcloud/numba,seibert/numba