build: migrate to flakybot (#141)

This PR was generated using Autosynth. 🌈

Synth log will be available here:
https://source.cloud.google.com/results/invocations/fe3713d3-5a91-469f-8d32-daadd024abff/targets

- [ ] To automatically regenerate this PR, check this box.

Source-Link: googleapis/synthtool@d1bb917

synth.metadata

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/python-api-core.git",
-        "sha": "5e8923f541b8482a05ab46554b108a2efcc615a9"
+        "sha": "786e9de706be03354d08ad98d1988eafb0628313"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "573f7655311b553a937f9123bee17bf78497db95"
+        "sha": "d1bb9173100f62c0cfc8f3138b62241e7f47ca6a"
       }
     }
   ],