Change Python 3.8-dev to 3.8 in Travis (#477)

Author: jtmiclat

.travis.yml

@@ -18,7 +18,7 @@ matrix:
       env: NOX_SESSION=test-3.6
     - python: 3.7
       env: NOX_SESSION=test-3.7
-    - python: 3.8-dev
+    - python: 3.8
       env: NOX_SESSION=test-3.8
       dist: bionic # Required to get OpenSSL 1.1.1+
 
@@ -37,9 +37,6 @@ matrix:
     # FIX
     # Some tests not yet resolved for Windows. (In progress)
     - os: windows
-    # Allow builds to fail when using '-dev' Python versions.
-    # We should still investigate failures periodically for new APIs.
-    - python: 3.8-dev
 
 install:
   - pip install --upgrade nox