Decode stdout during documentation build (ros-infrastructure#628)

I think that I'm seeing this because doc builds are happening with
python 3 on Fedora. Here is the error I'm seeing:

```
...
  File "./rosdep_doc_utils.py", line 38, in run
    nodes.literal_block(text=re.sub(escaped_capitalized_usage, '', out))
  File "/usr/lib64/python3.6/re.py", line 191, in sub
    return _compile(pattern, flags).sub(repl, string, count)
TypeError: cannot use a string pattern on a bytes-like object
```

doc/rosdep_doc_utils.py

@@ -35,14 +35,14 @@ def run(self):
                 [py, '-c', "from rosdep2.main import rosdep_main;rosdep_main(['-h'])"]
             )
             return [
-                nodes.literal_block(text=re.sub(escaped_capitalized_usage, '', out))
+                nodes.literal_block(text=re.sub(escaped_capitalized_usage, '', out.decode()))
             ]
         if 'install' in self.arguments:
             out = subprocess.check_output(
                 [py, '-c', "from rosdep2.main import rosdep_main;rosdep_main(['install', '-h'])"]
             )
             return [
-                nodes.literal_block(text=re.sub(escaped_capitalized_usage, '', out))
+                nodes.literal_block(text=re.sub(escaped_capitalized_usage, '', out.decode()))
             ]