PYTHON-2907 Fix up requirements files

.evergreen/test-cryptography-requirements.txt

@@ -0,0 +1,7 @@
+# cffi==1.14.3 was the last installable release on RHEL 6.2 with Python 3.4
+cffi==1.14.3;python_version=="3.4"
+cffi>=1.12.0,<2;python_version!="3.4"
+cryptography>=2,<3.4;python_version=="2.7"
+cryptography>=2,<2.9;python_version=="3.4"
+cryptography>=2,<3.3;python_version=="3.5"
+cryptography>=2;python_version>"3.5"

.evergreen/test-encryption-requirements.txt

@@ -1,9 +1,3 @@
-# cffi==1.14.3 was the last installable release on RHEL 6.2 with Python 3.4
-cffi==1.14.3;python_version=="3.4"
-cffi>=1.12.0,<2;python_version!="3.4"
-cryptography>=2,<3.4;python_version=="2.7"
-cryptography>=2,<2.9;python_version=="3.4"
-cryptography>=2,<3.3;python_version=="3.5"
-cryptography>=2;python_version>"3.5"
+-r test-cryptography-requirements.txt
 # boto3 is required by drivers-evergreen-tools/.evergreen/csfle/set-temp-creds.sh
 boto3<2

.evergreen/test-pyopenssl-requirements.txt

@@ -1,4 +1,4 @@
--r test-encryption-requirements.txt
+-r test-cryptography-requirements.txt
 pyopenssl>=17.2.0,<20;python_version=="3.4"
 pyopenssl>=17.2.0;python_version!="3.4"
 attrs<=20.3.0;python_version=="3.4"