reopen master for next dev cycle (#969)

diff --git a/src/OpenSSL/version.py b/src/OpenSSL/version.py
index 6ea3bef..2274aac 100644
--- a/src/OpenSSL/version.py
+++ b/src/OpenSSL/version.py
@@ -17,7 +17,7 @@
     "__version__",
 ]
 
-__version__ = "20.0.0"
+__version__ = "21.0.0.dev"
 
 __title__ = "pyOpenSSL"
 __uri__ = "https://pyopenssl.org/"