Replace all instances of EnvironmentError with OSError

According to the text just above https://docs.python.org/3/library/exceptions.html#EnvironmentError, `EnvironmentError` has been an alias of `OSError` since Python 3.3. Replacing it makes the code more consistent since `OSError` is used in other places in the code too.

kitty/child.py

@@ -51,7 +51,7 @@ else:
             try:
                 with open('/proc/' + x + '/stat', 'rb') as f:
                     raw = f.read().decode('utf-8')
-            except EnvironmentError:
+            except OSError:
                 continue
             try:
                 q = int(raw.split(' ', 5)[4])