Ensure that existing warning filters aren't reset by py_suppress_warnings
Ensure that existing warning filters aren't reset by py_suppress_warnings

Fixes:  https://github.com/rstudio/tensorflow/issues/158

rstudio/reticulate,terrytangyuan/reticulate,rstudio/reticulate,rstudio/reticulate,terrytangyuan/reticulate,rstudio/reticulate,terrytangyuan/reticulate,rstudio/reticulate