Log Message:
py-google-api-core: updated to 1.31.0

1.31.0 (2021-07-07)

Features
add ServiceUnavailable exception to polling retries

Bug Fixes
undprecate entity factory helpers

1.30.0 (2021-06-08)

Features
add iterator capability to paged iterators

1.29.0 (2021-06-02)

Features
HTTPIterator now accepts a page_size parameter to control page …

Documentation
fix broken links in multiprocessing.rst

1.28.0 (2021-05-20)

Bug Fixes
require google-auth>=1.25.0

Miscellaneous Chores

1.27.0 (2021-05-18)

Features
Add support for rest/ token in x-goog-api-client header
retry google.auth TransportError and requests ConnectionError