gyp: make StringIO work in ninja.py

Allow both Python 2 and 3 to access StringIO.

This fixes `./configure --ninja`, which was broken by
#29371.

See: #29371 (comment)

PR-URL: #29414
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>

tools/gyp/pylib/gyp/generator/ninja.py

@@ -20,7 +20,10 @@
 import gyp.msvs_emulation
 import gyp.MSVSUtil as MSVSUtil
 import gyp.xcode_emulation
-from io import StringIO
+try:
+  from cStringIO import StringIO
+except ImportError:
+  from io import StringIO
 
 from gyp.common import GetEnvironFallback
 import gyp.ninja_syntax as ninja_syntax