Fix is_conn_error() for Python 3.3+ change to socket.error

In Python 3.3+, `socket.error` is no longer a distinct exception.
It is - as the docs say - "A deprecated alias of OSError". This
means that this check:

`isinstance(e, socket.error)`

is effectively equivalent to:

`isinstance(e, OSError)`

This is a problem, because `requests.exceptions.ConnectionError`
(another exception type we handle later in `is_conn_error()`) is
a subclass of `OSError` - so on Python 3 we never actually reach
the block that's intended to handle that exception type. We hit
the `isinstance(e, socket.error)` block at the start instead, and
of course the exception doesn't have an `errno` attribute, so we
just return `False` at that point.

There are a few different ways we could fix this; this commit
does it by ditching the `isinstance` checks, and dropping the
shortcut `return False` bailout from the early block. We'll still
ultimately return `False` unless the error is actually one of the
other types we handle; it just costs a couple more `isinstance`
checks.

I don't think replacing the `isinstance socket.error` checks is
really necessary at all. We can just check for an `errno` attr,
and if there is one and it's one of the values we check for...
that seems safe enough to treat as a connection error.

This also changes the second check to be a check of `e2`, not
`e` - I'm *fairly* sure this is what's actually intended, and
the current code was a copy-paste error.

Fixes: #1192

Signed-off-by: Adam Williamson <awilliam@redhat.com>

koji/__init__.py

@@ -1978,11 +1978,13 @@ def is_cert_error(e):
 
 def is_conn_error(e):
     """Determine if an error seems to be from a dropped connection"""
-    if isinstance(e, socket.error):
-        if getattr(e, 'errno', None) in (errno.ECONNRESET, errno.ECONNABORTED, errno.EPIPE):
-            return True
-        # else
-        return False
+    # This is intended for the case where e is a socket error.
+    # as `socket.error` is just an alias for `OSError` in Python 3
+    # there is no value to an `isinstance` check here; let's just
+    # assume that if the exception has an 'errno' and it's one of
+    # these values, this is a connection error.
+    if getattr(e, 'errno', None) in (errno.ECONNRESET, errno.ECONNABORTED, errno.EPIPE):
+        return True
     if isinstance(e, six.moves.http_client.BadStatusLine):
         return True
     try:
@@ -1994,10 +1996,9 @@ def is_conn_error(e):
                 e3 = getattr(e2, 'args', [None, None])[1]
                 if isinstance(e3, six.moves.http_client.BadStatusLine):
                     return True
-            if isinstance(e2, socket.error):
-                # same check as unwrapped socket error
-                if getattr(e, 'errno', None) in (errno.ECONNRESET, errno.ECONNABORTED, errno.EPIPE):
-                    return True
+            # same check as unwrapped socket error
+            if getattr(e2, 'errno', None) in (errno.ECONNRESET, errno.ECONNABORTED, errno.EPIPE):
+                return True
     except (TypeError, AttributeError):
         pass
     # otherwise