Avoid unsafe use of pip (#3726)

* Always use latest version of conda. Ensure pip is always installed.

* pip -> python -m pip

ci/azure/install.yml

@@ -6,20 +6,22 @@ steps:
 - template: add-conda-to-path.yml
 
 - bash: |
+    conda update -y conda
     conda env create -n xarray-tests --file ${{ parameters.env_file }}
   displayName: Install conda dependencies
 
 - bash: |
     source activate xarray-tests
-    pip install -f https://7933911d6844c6c53a7d-47bd50c35cd79bd838daf386af554a83.ssl.cf2.rackcdn.com \
+    python -m pip install \
+        -f https://7933911d6844c6c53a7d-47bd50c35cd79bd838daf386af554a83.ssl.cf2.rackcdn.com \
         --no-deps \
         --pre \
         --upgrade \
         matplotlib \
         numpy \
         pandas \
         scipy
-    pip install \
+    python -m pip install \
         --no-deps \
         --upgrade \
         git+https://github.com/dask/dask \
@@ -33,7 +35,7 @@ steps:
 
 - bash: |
     source activate xarray-tests
-    pip install --no-deps -e .
+    python -m pip install --no-deps -e .
   displayName: Install xarray
 
 - bash: |

ci/requirements/py36-bare-minimum.yml

@@ -4,6 +4,7 @@ channels:
 dependencies:
   - python=3.6
   - coveralls
+  - pip
   - pytest
   - pytest-cov
   - pytest-env

ci/requirements/py36-min-nep18.yml

@@ -11,6 +11,7 @@ dependencies:
   - numpy=1.17
   - pandas=0.25
   - pint=0.9  # Actually not enough as it doesn't implement __array_function__yet!
+  - pip
   - pytest
   - pytest-cov
   - pytest-env