Remove usage of 'U' mode bit for opening files in python

The 'U' mode bit is removed in python 3.11. It has been
deprecated for a long time. The 'U' mode bit has no effect
so this change doesn't change any behavior.

See https://docs.python.org/3.11/whatsnew/3.11.html#changes-in-the-python-api

waflib/ConfigSet.py

@@ -312,7 +312,7 @@ def load(self, filename):
 		:type filename: string
 		"""
 		tbl = self.table
-		code = Utils.readf(filename, m='rU')
+		code = Utils.readf(filename, m='r')
 		for m in re_imp.finditer(code):
 			g = m.group
 			tbl[g(2)] = eval(g(3))

waflib/Context.py

@@ -266,7 +266,7 @@ def recurse(self, dirs, name=None, mandatory=True, once=True, encoding=None):
 				cache[node] = True
 				self.pre_recurse(node)
 				try:
-					function_code = node.read('rU', encoding)
+					function_code = node.read('r', encoding)
 					exec(compile(function_code, node.abspath(), 'exec'), self.exec_dict)
 				finally:
 					self.post_recurse(node)
@@ -662,7 +662,7 @@ def load_module(path, encoding=None):
 
 	module = imp.new_module(WSCRIPT_FILE)
 	try:
-		code = Utils.readf(path, m='rU', encoding=encoding)
+		code = Utils.readf(path, m='r', encoding=encoding)
 	except EnvironmentError:
 		raise Errors.WafError('Could not read the file %r' % path)