Rename chain_ex to add_ex_ctx to reflect upstream terminology

Rename chain_ex to add_ex_ctx to reflect upstream terminology

In the raise[1] docs Python uses chaining to refer to "from" (cause)
linkage, not implicit exception/finally (context) linkage, so adjust
our function name to match.  (Also preparation for compat code to
handle from linkage).

[1] https://docs.python.org/3/reference/simple_stmts.html#the-raise-statement.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>