Limited cryptography to < 3.2 for python 2.7

This is due to: pyca/cryptography#5359 (comment)

Dockerfile.ci

@@ -250,7 +250,7 @@ ENV AIRFLOW_CONSTRAINTS_URL=${AIRFLOW_CONSTRAINTS_URL}
 
 # By changing the CI build epoch we can force reinstalling Airflow from the current master
 # It can also be overwritten manually by setting the AIRFLOW_CI_BUILD_EPOCH environment variable.
-ARG AIRFLOW_CI_BUILD_EPOCH="3"
+ARG AIRFLOW_CI_BUILD_EPOCH="4"
 ENV AIRFLOW_CI_BUILD_EPOCH=${AIRFLOW_CI_BUILD_EPOCH}
 
 ARG AIRFLOW_PRE_CACHED_PIP_PACKAGES="true"

setup.py

@@ -219,7 +219,12 @@ def write_version(filename=os.path.join(*[my_dir, "airflow", "git_version"])):
 cloudant = [
  'cloudant>=0.5.9,<2.0',
 ]
-crypto = ['cryptography>=0.9.3']
+crypto = [
+ # Cryptography 3.2 for python 2.7 is broken
+ # https://github.com/pyca/cryptography/issues/5359#issuecomment-727622403
+ 'cryptography>=0.9.3,<3.2; python_version<"3.0"',
+ 'cryptography>=0.9.3;python_version>="3.0"',
+]
 dask = [
  'distributed>=1.17.1, <2',
 ]