Pin Python version in environment

It seems that `conda env update` is overriding the match specification
from the previous `python` install command. To fix this, we create the
environment empty and pin the `python` version in the `pinned` file.
This should be respected by `conda env update`.

.appveyor.yml

@@ -64,9 +64,10 @@ install:
     - cmd: conda config --get
 
     # Create the test environment.
-    - cmd: conda create -y -n testenv python="%CONDA_PY%"
-    - cmd: conda env update -n testenv --file environment_ci.yml
+    - cmd: conda create -y -n testenv
     - cmd: conda activate testenv
+    - cmd: echo "python %CONDA_PY%.*" >> "%CONDA_PREFIX%\\conda-meta\\pinned"
+    - cmd: conda env update -n testenv --file environment_ci.yml
 
 # Skip .NET project specific build phase.
 build: off

.travis.yml

@@ -29,9 +29,10 @@ install:
   - conda clean -tipsy
 
   # Create the test environment.
-  - conda create -y -n testenv python="${PYVER}"
-  - conda env update -n testenv --file environment_ci.yml
+  - conda create -y -n testenv
   - conda activate testenv
+  - echo "python ${PYVER}.*" >> "${CONDA_PREFIX}/conda-meta/pinned"
+  - conda env update -n testenv --file environment_ci.yml
 
 # command to run tests, e.g. python setup.py test
 script: