[3.6] bpo-30375: Correct the stacklevel of regex compiling warnings. (GH-1595) (...

[3.6] bpo-30375: Correct the stacklevel of regex compiling warnings. (GH-1595) (#1604)

Warnings emitted when compile a regular expression now always point
to the line in the user code.  Previously they could point into inners
of the re module if emitted from inside of groups or conditionals..
(cherry picked from commit c7ac7280c321b3c1679fe5f657a6be0f86adf173)