Fix startup on python 3 if run through uwsgi

uwsgi options are returned as bytestring, so this avoids:
```
Traceback (most recent call last):
  File "lib/galaxy/webapps/galaxy/buildapp.py", line 12, in <module>
    import galaxy.app
  File "lib/galaxy/app.py", line 8, in <module>
    import galaxy.model
  File "lib/galaxy/model/__init__.py", line 45, in <module>
    import galaxy.exceptions
  File "lib/galaxy/exceptions/__init__.py", line 19, in <module>
    from ..exceptions import error_codes
  File "lib/galaxy/exceptions/error_codes.py", line 9, in <module>
    from galaxy.util import unicodify
  File "lib/galaxy/util/__init__.py", line 1708, in <module>
    executable = _executable()
  File "lib/galaxy/util/__init__.py", line 1700, in _executable
    exe = os.path.join(virtualenv, 'bin', 'python')
  File "/Users/mvandenb/src/galaxy/.venv3/lib/python3.6/posixpath.py", line 94, in join
    genericpath._check_arg_types('join', a, *p)
  File "/Users/mvandenb/src/galaxy/.venv3/lib/python3.6/genericpath.py", line 151, in _check_arg_types
    raise TypeError("Can't mix strings and bytes in path components") from None
TypeError: Can't mix strings and bytes in path components
```

Broken in https://github.com/galaxyproject/galaxy/commit/891daf4f82eb45c692f3fc5542cea35f94e10118

lib/galaxy/util/__init__.py

@@ -1692,7 +1692,7 @@ def _executable():
         if uwsgi is not None:
             for name in ('home', 'virtualenv', 'venv', 'pyhome'):
                 if name in uwsgi.opt:
-                    virtualenv = uwsgi.opt[name]
+                    virtualenv = unicodify(uwsgi.opt[name])
                     break
         if virtualenv is None and 'VIRTUAL_ENV' in os.environ:
             virtualenv = os.environ['VIRTUAL_ENV']