use logging.warning() instead of deprecated logging.warn()

functionality is identical, the latter is deprecated:
https://docs.python.org/3/library/logging.html#logging.Logger.warning

Lib/fontTools/merge.py

@@ -388,7 +388,7 @@ def merge(self, m, tables):
 					# char previously mapped to oldgid but already remapped to a different gid,
 					# save new gid as an alternate
 					# TODO: try harder to save these
-					log.warn("Dropped mapping from codepoint %#06X to glyphId '%s'", uni, gid)
+					log.warning("Dropped mapping from codepoint %#06X to glyphId '%s'", uni, gid)
 
 	cmapBmpOnly = {uni: gid for uni,gid in cmap.items() if uni <= 0xFFFF}
 	self.tables = []

Lib/fontTools/mtiLib/__init__.py

@@ -1073,7 +1073,7 @@ class Tokenizer(object):
 		if len(line) == 1 and not line[0]:
 			del line[0]
 		if line and not line[-1]:
-			log.warn('trailing tab found on line %d: %s' % (self.lineno, self.line))
+			log.warning('trailing tab found on line %d: %s' % (self.lineno, self.line))
 			while line and not line[-1]:
 				del line[-1]
 		return line

Lib/fontTools/varLib/__init__.py

@@ -218,7 +218,7 @@ def _add_gvar(font, model, master_ttfs, tolerance=.5):
 		allControls = [d[1] for d in allData]
 		control = allControls[0]
 		if (any(c != control for c in allControls)):
-			log.warn("glyph %s has incompatible masters; skipping" % glyph)
+			log.warning("glyph %s has incompatible masters; skipping" % glyph)
 			continue
 		del allControls