ensure cythonize numcodecs for py37 on appveyor

Author: alimanfoo

appveyor.yml

@@ -53,6 +53,7 @@ build: off
 test_script:
   - "%CMD_IN_ENV% python -m pip install -U pip setuptools wheel"
   - "%CMD_IN_ENV% python -m pip install numpy==%NUMPY_VERSION%"
+  - "%CMD_IN_ENV% python -m pip install cython==0.29"
   - "%CMD_IN_ENV% python -m pip install -v --no-binary=numcodecs numcodecs==0.5.5"
   - "%CMD_IN_ENV% python -m pip install -rrequirements_dev.txt"
   - "%CMD_IN_ENV% python setup.py install"