Add "Cancun" fork to ForkName

.circleci/config.yml

@@ -22,7 +22,9 @@ common: &common
           - cache-v1-{{ arch }}-{{ .Environment.CIRCLE_JOB }}-{{ checksum "setup.py" }}-{{ checksum "tox.ini" }}
     - run:
         name: install dependencies
-        command: python -m pip install --user tox
+        command: |
+          python -m pip install --upgrade pip
+          python -m pip install tox
     - run:
         name: run tox
         command: python -m tox -r
@@ -50,7 +52,9 @@ windows_steps: &windows_steps
           - cache-v1-{{ arch }}-{{ .Environment.CIRCLE_JOB }}-{{ checksum "setup.py" }}-{{ checksum "tox.ini" }}
     - run:
         name: install dependencies
-        command: python -m pip install --user tox
+        command: |
+          python -m pip install --upgrade pip
+          python -m pip install tox
     - run:
         name: run tox
         command: python -m tox -r

eth_typing/enums.py

@@ -14,3 +14,4 @@ class ForkName:
     GrayGlacier = "GrayGlacier"
     Paris = "Paris"
     Shanghai = "Shanghai"
+    Cancun = "Cancun"

newsfragments/45.feature.rst

@@ -0,0 +1 @@
+Add ``Cancun`` to ``ForkName`` enum.

newsfragments/45.internal.rst

@@ -0,0 +1 @@
+For CircleCI builds, update ``pip`` and pip install ``tox`` under sys instead of ``--user`` to avoid ``virtualenv`` versioning issues.