Add 3.5.3 requirement to setup.py's description

Author: smurfix

README.rst

@@ -26,7 +26,8 @@
 **Trio-Asyncio** is a re-implementation of the ``asyncio`` mainloop on top of
 Trio.
 
-Trio-Asyncio requires Python 3.6 or 3.7. It might work on Python 3.5.
+Trio-Asyncio requires Python 3.6 or 3.7. It should work on Python 3.5.3, but
+that's not tested, as the test suite requires 3.6.
 
 +++++++++++
  Rationale

setup.py

@@ -43,6 +43,8 @@
 code (asyncio: ~8000) but passes the complete Python 3.6 test suite with no
 errors.
 
+``trio_asyncio`` requires Python 3.5.3 or better.
+
 Author
 ======