Fixed compile error in issue #22410. The _locale module is optional.

Lib/re.py

@@ -122,7 +122,10 @@ This module also defines an exception 'error'.
 import sys
 import sre_compile
 import sre_parse
+try:
     import _locale
+except ImportError:
+    _locale = None
 
 # public symbols
 __all__ = [ "match", "fullmatch", "search", "sub", "subn", "split", "findall",
@@ -293,6 +296,8 @@ def _compile(pattern, flags):
         if len(_cache) >= _MAXCACHE:
             _cache.clear()
         if p.flags & LOCALE:
+            if not _locale:
+                return p
             loc = _locale.setlocale(_locale.LC_CTYPE)
         else:
             loc = None