generate .pyi files for proxied classes

generate .pyi files for proxied classes

Stub .pyi files have been added for the "dynamically"
generated Alembic modules ``alembic.op`` and ``alembic.context``, which
include complete function signatures and docstrings, so that the functions
in these namespaces will have both IDE support (vscode, pycharm, etc) as
well as support for typing tools like Mypy. The files themselves are
statically generated from their source functions within the source tree.

Still not available is sphinx autodoc from the .pyi file.  so
we might want to further modify this to write into the .py directly.

if we do *that*, we could almost get rid of ModuleClsProxy and
just generate proxying methods fully in op.py.  however, this won't work
for the "extending ops" use case.

Change-Id: I3f084040de25ed3f8e92a4e9d0bb83d69b78eac6