Use consistent method signature for Alias.self_group()

Use consistent method signature for Alias.self_group()

Fixed bug where the use of an :class:`.Alias` object in a column
context would raise an argument error when it tried to group itself
into a parenthesized expression.   Using :class:`.Alias` in this way
is not yet a fully supported API, however it applies to some end-user
recipes and may have a more prominent role in support of some
future Postgresql features.

Change-Id: I81717e30416e0350f08d1e022c3d84656e0a9735
Fixes: #3939