Restore Python 3.6 support

Author: kbattocchi

azure-pipelines-steps.yml

@@ -6,7 +6,7 @@
 parameters:
   package: '-e .'
   images: ['ubuntu-18.04', 'macOS-10.15', 'windows-2019']
-  versions: ['3.7', '3.8', '3.9']
+  versions: ['3.6', '3.7', '3.8', '3.9']
   job: 
     job: Job
 

azure-pipelines.yml

@@ -198,7 +198,7 @@ jobs:
       condition: eq(dependencies.EvalChanges.outputs['output.testCode'], 'True')
       displayName: 'Run tests (main)'
       steps:
-      - script: 'pip install pytest pytest-runner && python setup.py pytest'
+      - script: 'pip install pytest pytest-runner "coverage<6.4.1;python_version==''3.6''" "coverage;python_version>''3.6''" && python setup.py pytest'
         displayName: 'Unit tests'
         env:
           PYTEST_ADDOPTS: '-m "not (notebook or automl or dml or serial or cate_api)" -n 2'
@@ -225,7 +225,7 @@ jobs:
       condition: eq(dependencies.EvalChanges.outputs['output.testCode'], 'True')
       displayName: 'Run tests (DML)'
       steps:
-      - script: 'pip install pytest pytest-runner && python setup.py pytest'
+      - script: 'pip install pytest pytest-runner "coverage<6.4.1;python_version==''3.6''" "coverage;python_version>''3.6''" && python setup.py pytest'
         displayName: 'Unit tests'
         env:
           PYTEST_ADDOPTS: '-m "dml"'
@@ -254,7 +254,7 @@ jobs:
       condition: eq(dependencies.EvalChanges.outputs['output.testCode'], 'True')
       displayName: 'Run tests (Serial)'
       steps:
-      - script: 'pip install pytest pytest-runner && python setup.py pytest'
+      - script: 'pip install pytest pytest-runner "coverage<6.4.1;python_version==''3.6''" "coverage;python_version>''3.6''" && python setup.py pytest'
         displayName: 'Unit tests'
         env:
           PYTEST_ADDOPTS: '-m "serial" -n 1'
@@ -281,7 +281,7 @@ jobs:
       condition: eq(dependencies.EvalChanges.outputs['output.testCode'], 'True')
       displayName: 'Run tests (Other)'
       steps:
-      - script: 'pip install pytest pytest-runner'
+      - script: 'pip install pytest pytest-runner "coverage<6.4.1;python_version==''3.6''" "coverage;python_version>''3.6''"'
         displayName: 'Install pytest'
       - script: 'python setup.py pytest'
         displayName: 'CATE Unit tests'

setup.cfg

@@ -20,6 +20,7 @@ project_urls =
     Source Code=https://github.com/Microsoft/EconML
     Documentation=https://econml.azurewebsites.net/
 classifiers =
+    Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9