deps: Upgrade tornado to v6.4

  `tornado` 5.1.1 uses some `ssl` methods (specifically
  `ssl.match_hostname`) which have been deprecated in Python 3.10 and
  later removed (see python/cpython#94199).
  This is preventing us from even installing `tornado` v5.1.1 as there
  are checks in the setup.
  These have been removed in `tornado` 6.0 though
  (tornadoweb/tornado@fa6a7ff)
  so we can install this dependency using Python 3.12 if we use
  `tornado` v6.x.

  v6.4 is the latest stable version at this moment.

requirements.txt

@@ -22,7 +22,7 @@ requests==2.20.0
 sh==1.12.14
 six==1.11.0
 terminaltables==3.1.0
-tornado==5.1.1
+tornado==6.4
 transifex-client
 tx==0.0.1
 Unidecode==1.0.22