traivs: drop pypy3 test

Signed-off-by: Hiroshi Miura <miurahr@linux.com>

.travis.yml

@@ -1,7 +1,6 @@
 language: python
 dist: bionic
 sudo: false
-cache: pip
 matrix:
   include:
     - python: 3.8
@@ -10,15 +9,9 @@ matrix:
       env: TOXENV=docs PYTEST_NO_COVERAGE=1
     - python: 3.8
       env: TOXENV=mypy PYTEST_NO_COVERAGE=1
-    - python: pypy3.6-7.2.0
-      env: TOXENV=pypy3
 
 install:
   - pip install -U pip
-  - pip install tox tox-travis coveralls
+  - pip install tox tox-travis
+
 script: tox
-after_success:
-  - |
-    if [[ "$PYTEST_NO_COVERAGE" != 1 ]]; then
-      coveralls
-    fi