Fix compiler warning in sre.c

sre_lower_ascii() is called by _sre_ascii_tolower_impl() with a
signed "int".

Author: vstinner

Modules/_sre/sre.c

@@ -106,7 +106,7 @@ static const char copyright[] =
 #define SRE_IS_WORD(ch)\
     ((ch) <= 'z' && (Py_ISALNUM(ch) || (ch) == '_'))
 
-static unsigned int sre_lower_ascii(unsigned int ch)
+static unsigned int sre_lower_ascii(int ch)
 {
     return ((ch) < 128 ? Py_TOLOWER(ch) : ch);
 }