[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)
diff --git a/Lib/sre_parse.py b/Lib/sre_parse.py
index e1c7496..608f9a2 100644
--- a/Lib/sre_parse.py
+++ b/Lib/sre_parse.py
@@ -404,7 +404,7 @@
         pass
     raise source.error("bad escape %s" % escape, len(escape))
 
-def _parse_sub(source, state, verbose, nested=True):
+def _parse_sub(source, state, verbose, nested):
     # parse an alternation: a|b|c
 
     items = []
@@ -412,7 +412,8 @@
     sourcematch = source.match
     start = source.tell()
     while True:
-        itemsappend(_parse(source, state, verbose, not nested and not items))
+        itemsappend(_parse(source, state, verbose, nested + 1,
+                           not nested and not items))
         if not sourcematch("|"):
             break
 
@@ -454,10 +455,10 @@
     subpattern.append((BRANCH, (None, items)))
     return subpattern
 
-def _parse_sub_cond(source, state, condgroup, verbose):
-    item_yes = _parse(source, state, verbose)
+def _parse_sub_cond(source, state, condgroup, verbose, nested):
+    item_yes = _parse(source, state, verbose, nested + 1)
     if source.match("|"):
-        item_no = _parse(source, state, verbose)
+        item_no = _parse(source, state, verbose, nested + 1)
         if source.next == "|":
             raise source.error("conditional backref with more than two branches")
     else:
@@ -466,7 +467,7 @@
     subpattern.append((GROUPREF_EXISTS, (condgroup, item_yes, item_no)))
     return subpattern
 
-def _parse(source, state, verbose, first=False):
+def _parse(source, state, verbose, nested, first=False):
     # parse a simple pattern
     subpattern = SubPattern(state)
 
@@ -692,7 +693,7 @@
                         lookbehindgroups = state.lookbehindgroups
                         if lookbehindgroups is None:
                             state.lookbehindgroups = state.groups
-                    p = _parse_sub(source, state, verbose)
+                    p = _parse_sub(source, state, verbose, nested + 1)
                     if dir < 0:
                         if lookbehindgroups is None:
                             state.lookbehindgroups = None
@@ -739,7 +740,7 @@
                                     source.string[:20],  # truncate long regexes
                                     ' (truncated)' if len(source.string) > 20 else '',
                                 ),
-                                DeprecationWarning, stacklevel=7
+                                DeprecationWarning, stacklevel=nested + 6
                             )
                         if (state.flags & SRE_FLAG_VERBOSE) and not verbose:
                             raise Verbose
@@ -757,11 +758,11 @@
                 except error as err:
                     raise source.error(err.msg, len(name) + 1) from None
             if condgroup:
-                p = _parse_sub_cond(source, state, condgroup, verbose)
+                p = _parse_sub_cond(source, state, condgroup, verbose, nested + 1)
             else:
                 sub_verbose = ((verbose or (add_flags & SRE_FLAG_VERBOSE)) and
                                not (del_flags & SRE_FLAG_VERBOSE))
-                p = _parse_sub(source, state, sub_verbose)
+                p = _parse_sub(source, state, sub_verbose, nested + 1)
             if not source.match(")"):
                 raise source.error("missing ), unterminated subpattern",
                                    source.tell() - start)
@@ -851,7 +852,7 @@
     pattern.str = str
 
     try:
-        p = _parse_sub(source, pattern, flags & SRE_FLAG_VERBOSE, False)
+        p = _parse_sub(source, pattern, flags & SRE_FLAG_VERBOSE, 0)
     except Verbose:
         # the VERBOSE flag was switched on inside the pattern.  to be
         # on the safe side, we'll parse the whole thing again...
@@ -859,7 +860,7 @@
         pattern.flags = flags | SRE_FLAG_VERBOSE
         pattern.str = str
         source.seek(0)
-        p = _parse_sub(source, pattern, True, False)
+        p = _parse_sub(source, pattern, True, 0)
 
     p.pattern.flags = fix_flags(str, p.pattern.flags)