crash interpolating bytes with dictionary

[benjaminp]

$ cat example.py
print(b'%(x)s' % {b'x': b'data'})
$ python3 example.py
b'data'
$ mypy example.py --show-traceback
mypy example.py  --show-traceback
example.py:1: error: INTERNAL ERROR -- Please try using mypy master on Github:
https://mypy.rtfd.io/en/latest/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 0.720
Traceback (most recent call last):
  File "mypy/checkexpr.py", line 3404, in accept
  File "mypy/nodes.py", line 1708, in accept
  File "mypy/checkexpr.py", line 1891, in visit_op_expr__ExpressionVisitor_glue
  File "mypy/checkexpr.py", line 1906, in visit_op_expr
  File "mypy/checkstrformat.py", line 97, in check_str_interpolation
  File "mypy/checkstrformat.py", line 178, in check_mapping_str_interpolation
TypeError: StrExpr object expected
example.py:1: : note: use --pdb to drop into pdb