Merge pull request #15965 from grpc/v1.13.x

Upmerge v1.13.x (Python3.7 macOS/Windows 64-bit targets)

tools/internal_ci/macos/grpc_build_artifacts.sh

@@ -25,6 +25,7 @@ python2.7 -m pip install cython setuptools wheel
 python3.4 -m pip install cython setuptools wheel
 python3.5 -m pip install cython setuptools wheel
 python3.6 -m pip install cython setuptools wheel
+python3.7 -m pip install cython setuptools wheel
 
 # needed to build ruby artifacts
 time bash tools/distrib/build_ruby_environment_macos.sh

tools/run_tests/artifacts/artifact_targets.py

@@ -373,6 +373,7 @@ def targets():
         PythonArtifact('macos', 'x64', 'python3.4'),
         PythonArtifact('macos', 'x64', 'python3.5'),
         PythonArtifact('macos', 'x64', 'python3.6'),
+        PythonArtifact('macos', 'x64', 'python3.7'),
         PythonArtifact('windows', 'x86', 'Python27_32bits'),
         PythonArtifact('windows', 'x86', 'Python34_32bits'),
         PythonArtifact('windows', 'x86', 'Python35_32bits'),
@@ -381,6 +382,7 @@ def targets():
         PythonArtifact('windows', 'x64', 'Python34'),
         PythonArtifact('windows', 'x64', 'Python35'),
         PythonArtifact('windows', 'x64', 'Python36'),
+        PythonArtifact('windows', 'x64', 'Python37'),
         RubyArtifact('linux', 'x64'),
         RubyArtifact('macos', 'x64'),
         PHPArtifact('linux', 'x64'),