MAINT pin the version of Python in azure (scikit-learn#15378)

Author: glemaitre

azure-pipelines.yml

@@ -32,7 +32,8 @@ jobs:
       # It runs tests requiring pandas and PyAMG.
       pylatest_pip_openblas_pandas:
         DISTRIB: 'conda-pip-latest'
-        PYTHON_VERSION: '*'
+        # FIXME: pinned until SciPy wheels are available for Pyhon 3.8
+        PYTHON_VERSION: '3.7'
         PYTEST_VERSION: '4.6.2'
         COVERAGE: 'true'
         CHECK_PYTEST_SOFT_DEPENDENCY: 'true'