[FIX] requirements: limit the version of cryptography

It happens that pyopenssl==19.0.0 is incompatible with cryptography>=37.0.0,
but in pyopenssl requirements, they have lower bound limit for
cryptography but not an upper bound.

requirements.txt

@@ -1,6 +1,6 @@
 # generated from manifests external_dependencies
 chardet
-cryptography
+cryptography==2.6.1
 pycountry
 pycryptodome
 qrcode