windows-1809: Update python to 3.7.6

windows-1809/base/Dockerfile

@@ -19,7 +19,7 @@ RUN [Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tl
         curl \
         7zip \
         wixtoolset; \
-    Must-Run choco install -y python --version 3.7.5; \
+    Must-Run choco install -y python --version 3.7.6; \
     $env:PATH = "\"$([Environment]::GetEnvironmentVariable('PATH', [EnvironmentVariableTarget]::User));$([Environment]::GetEnvironmentVariable('PATH', [EnvironmentVariableTarget]::Machine))\""; \
     Must-Run python -m pip --no-cache-dir install --no-warn-script-location wheel; \
     Must-Run choco install -y visualstudio2017buildtools; \