[deps] Set min cryptography version to 3.4, allow any higher 3.x

requirements.txt

@@ -1,6 +1,6 @@
 django>=2.2.0,<3.2.0
 openwisp-utils~=0.7.2
 jsonfield>=3.1.0,<4.0.0
-cryptography~=3.2.0
+cryptography~=3.4
 pyopenssl>=17.5.0,<20.1.0
 swapper~=1.1.0