setup.py: rename _version.py to _black_version.py (#1089)

In #1082, _version.py was renamed to _black_version.py, but the
py_modules line in setup.py wasn't changed, which meant that when
installed from source, running it failed with something like:

```
Traceback (most recent call last):
  File "~/.pyenv/versions/3.6.5/bin/black", line 5, in <module>
    from black import patched_main
  File "~/.pyenv/versions/3.6.5/lib/python3.6/site-packages/black.py", line 55, in <module>
    from _black_version import version as __version__
ModuleNotFoundError: No module named '_black_version'
```