simplify setext heading regex: [=]+ → =+ and [-]+ → -+

The character class form [=]+ and [=]+ are equivalent to =+
and =+ respectively, but the bare form is cleaner per mitya57's
review suggestion on PR Python-Markdown#1606.

Author: Jah-yee

markdown/blockprocessors.py

@@ -494,7 +494,7 @@ class SetextHeaderProcessor(BlockProcessor):
     """ Process Setext-style Headers. """
 
     # Detect Setext-style header. Must be first 2 lines of block.
-    RE = re.compile(r'^.*?\n(?:[=]+|[-]+)[ ]*(\n|$)', re.MULTILINE)
+    RE = re.compile(r'^.*?\n(?:=+|-+)[ ]*(\n|$)', re.MULTILINE)
 
     def test(self, parent: etree.Element, block: str) -> bool:
         return bool(self.RE.match(block))