add Travis testing for PyPy 5.7.1

Author: mattsb42-aws

.travis.yml

@@ -65,6 +65,17 @@ matrix:
       env: TOXENV=pypy-pycrypto-norsa
     - python: pypy-5.3.1
       env: TOXENV=pypy-compatibility
+    # PyPy 5.7.1
+    - python: pypy-5.7.1
+      env: TOXENV=pypy-base
+    - python: pypy-5.7.1
+      env: TOXENV=pypy-cryptography-only
+    - python: pypy-5.7.1
+      env: TOXENV=pypy-pycryptodome-norsa
+    - python: pypy-5.7.1
+      env: TOXENV=pypy-pycrypto-norsa
+    - python: pypy-5.7.1
+      env: TOXENV=pypy-compatibility
 # matrix:
 #   include:
 #     - python: 3.6