Clean-up test requirements

after security warning

tests/requirements-tests.txt

@@ -1,30 +1,9 @@
 #
-# This file is autogenerated by pip-compile
-# To update, run:
-#
-#    pip-compile --output-file tests/requirements-tests.txt setup.py tests/requirements-tests.in
-#
-apipkg==1.4               # via execnet
-attrs==18.1.0             # via pytest
-certifi==2018.4.16        # via requests
-chardet==3.0.4            # via requests
-coverage==4.5.1           # via pytest-cov
-execnet==1.5.0            # via pytest-cache
-idna==2.6                 # via requests
-mccabe==0.6.1             # via pytest-mccabe
-more-itertools==4.1.0     # via pytest
-pep8==1.7.1               # via pytest-pep8
-pluggy==0.6.0             # via pytest
-py==1.5.3                 # via pytest
-pyflakes==1.6.0           # via pytest-flakes
-pytest-cache==1.0         # via pytest-flakes, pytest-mccabe, pytest-pep8
-pytest-cov==2.5.1
-pytest-env==0.6.2
-pytest-flakes==2.0.0
-pytest-mccabe==0.1
-pytest-pep8==1.0.6
-pytest-runner==4.2
-pytest==3.5.1
-requests==2.18.4
-six==1.11.0               # via more-itertools, pytest
-urllib3==1.22             # via requests
+pytest-cov
+pytest-env
+pytest-flakes
+pytest-mccabe
+pytest-pep8
+pytest-runner
+pytest
+requests