temporarily pin pytest-cov to previous version

to work around 'No data to combine error'

The latest pytest-cov 2.6.0 seems to be running coverage combine itself:
pytest-dev/pytest-cov#222

we may revert this later if we find a better solution

.appveyor.yml

@@ -3,7 +3,7 @@ image: Visual Studio 2017
 environment:
   global:
     CIBW_SKIP: cp33-* cp34-* cp35-* cp37-* cp27-win_amd64 cp36-win_amd64
-    CIBW_TEST_REQUIRES: pytest-cov codecov subprocess32
+    CIBW_TEST_REQUIRES: pytest-cov==2.5.1 codecov subprocess32
     CIBW_TEST_COMMAND: cd {project} && pytest --cov -v && coverage combine && codecov && pip uninstall --yes afdko
     TWINE_USERNAME: afdko-travis
     # Note: TWINE_PASSWORD is set in AppVeyor settings

.travis.yml

@@ -5,7 +5,7 @@ env:
     - CIBW_SKIP="cp34-* cp35-* cp37-* *i686"
     - CIBW_BEFORE_BUILD_LINUX="yum install -y glibc-devel.i386 libgcc_s.so.1"
     - CIBW_ENVIRONMENT_LINUX="CODECOV_TOKEN=e0067bac-3ded-4432-83c8-667217c76660"
-    - CIBW_TEST_REQUIRES="pytest-cov codecov subprocess32"
+    - CIBW_TEST_REQUIRES="pytest-cov==2.5.1 codecov subprocess32"
     - CIBW_TEST_COMMAND="cd {project} && pytest --cov -v && coverage combine && codecov && pip uninstall --yes afdko"
     - TWINE_USERNAME="afdko-travis"
     # Note: TWINE_PASSWORD is set in Travis settings

requirements-dev.txt

@@ -3,7 +3,7 @@ cpplint>=1.3.0
 cython>=0.28.3
 flake8>=3.5.0
 pip>=10.0.1
-pytest-cov>=2.5.1
+pytest-cov==2.5.1
 setuptools>=39.2.0
 setuptools_scm>=2.1.0
 subprocess32>=3.5.0