Fix Travis CI build error

The error may be due to z4r/python-coveralls#66

Solution: drop CI support for Python <= 3.5, except 2.7, and apply a workaround for pypy.

.travis.yml

@@ -5,10 +5,6 @@ matrix:
       python: 2.7
     - arch: amd64
       python: 2.7
-    - arch: arm64
-      python: 3.5
-    - arch: amd64
-      python: 3.5
     - arch: arm64
       python: 3.7
     - arch: amd64
@@ -19,8 +15,14 @@ matrix:
       python: 3.8
     - arch: amd64
       python: pypy
+      before_install:
+        # Workaround for dependency errors, probably due to https://github.com/z4r/python-coveralls/issues/66
+      - pip install --upgrade --force-reinstall pytest-cov==2.5.0 
     - arch: amd64
       python: pypy3
+      before_install:
+        # Workaround for dependency errors, probably due to https://github.com/z4r/python-coveralls/issues/66
+        - pip install --upgrade --force-reinstall pytest-cov==2.5.0 
     - arch: arm64
       python: 2.7
       env: COVERAGE='on'
@@ -33,14 +35,6 @@ matrix:
     - arch: amd64
       python: 3.6
       env: COVERAGE='on'
-    - arch: arm64
-      python: 3.4
-      before_install:
-        - pip install --upgrade --force-reinstall setuptools
-    - arch: amd64
-      python: 3.4
-      before_install:
-        - pip install --upgrade --force-reinstall setuptools
 install:
   - pip install -U -e .[develop]
 script: