chore: pin versions of dependencies for compatibility with Python 3.6 (…

…#1588) (#966)

* chore: pin versions of dependencies for compatibility with Python 3.6

* fix path of requirements file in synthtool
Source-Link: googleapis/synthtool@69cdb47
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-java:latest@sha256:6566dc8226f20993af18e5a4e7a2b1ba85a292b02dedb6a1634cf10e1b418fa5

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
Co-authored-by: Diego Alonso Marquez Palacios <[email protected]>