Add Python3.7 32bit build on Windows

tools/internal_ci/windows/grpc_build_artifacts.bat

@@ -18,6 +18,7 @@ rename C:\Python27_32bit Python27_32bits
 rename C:\Python34_32bit Python34_32bits
 rename C:\Python35_32bit Python35_32bits
 rename C:\Python36_32bit Python36_32bits
+rename C:\Python37_32bit Python37_32bits
 
 @rem enter repo root
 cd /d %~dp0\..\..\..

tools/run_tests/artifacts/artifact_targets.py

@@ -378,6 +378,7 @@ def targets():
         PythonArtifact('windows', 'x86', 'Python34_32bits'),
         PythonArtifact('windows', 'x86', 'Python35_32bits'),
         PythonArtifact('windows', 'x86', 'Python36_32bits'),
+        PythonArtifact('windows', 'x86', 'Python37_32bits'),
         PythonArtifact('windows', 'x64', 'Python27'),
         PythonArtifact('windows', 'x64', 'Python34'),
         PythonArtifact('windows', 'x64', 'Python35'),