Cleanup README (#650)

Author: asvetlov

README.rst

@@ -57,25 +57,12 @@ Installation
 
    $ pip install aiosignal
 
-The library requires Python 3.8 or newer.
-
 
 Documentation
 =============
 
 https://aiosignal.readthedocs.io/
 
-Communication channels
-======================
-
-*gitter chat* https://gitter.im/aio-libs/Lobby
-
-Requirements
-============
-
-- Python >= 3.8
-- frozenlist >= 1.0.0
-
 License
 =======