Change python-future to 0.15.2 (again)

Merges https://github.com/espressif/esp-idf/pull/2492

requirements.txt

@@ -6,5 +6,5 @@ setuptools
 # Please keep it as the first item of this list.
 #
 pyserial>=3.0
-future>=0.16.0
+future>=0.15.2
 cryptography