chore: migrate default branch from master to main

[parthea]

Towards googleapis/google-cloud-python#10579

[plamut]

LGTM. Not sure why the OwlBot check failed, though.

[parthea]

It appears to be a transient error. The error from CI is

  Installing build dependencies: finished with status 'done'
  Getting requirements to build wheel: started
  Getting requirements to build wheel: finished with status 'done'
    Preparing wheel metadata: started
    Preparing wheel metadata: finished with status 'error'
    ERROR: Command errored out with exit status 1:
     command: /tmpfs/src/github/google-resumable-media-python/.nox/blacken/bin/python /tmpfs/src/github/google-resumable-media-python/.nox/blacken/lib/python3.8/site-packages/pip/_vendor/pep517/_in_process.py prepare_metadata_for_build_wheel /tmp/tmp42b1v5xx
         cwd: /tmp/pip-install-heiew4z0/black
    Complete output (30 lines):
    Traceback (most recent call last):
      File "/tmpfs/src/github/google-resumable-media-python/.nox/blacken/lib/python3.8/site-packages/pip/_vendor/pep517/_in_process.py", line 280, in <module>
        main()
      File "/tmpfs/src/github/google-resumable-media-python/.nox/blacken/lib/python3.8/site-packages/pip/_vendor/pep517/_in_process.py", line 263, in main
        json_out['return_val'] = hook(**hook_input['kwargs'])
      File "/tmpfs/src/github/google-resumable-media-python/.nox/blacken/lib/python3.8/site-packages/pip/_vendor/pep517/_in_process.py", line 133, in prepare_metadata_for_build_wheel
        return hook(metadata_directory, config_settings)
      File "/tmp/pip-build-env-b2d02oya/overlay/lib/python3.8/site-packages/setuptools/build_meta.py", line 166, in prepare_metadata_for_build_wheel
        self.run_setup()
      File "/tmp/pip-build-env-b2d02oya/overlay/lib/python3.8/site-packages/setuptools/build_meta.py", line 150, in run_setup
        exec(compile(code, __file__, 'exec'), locals())
      File "setup.py", line 48, in <module>
        setup(
      File "/tmp/pip-build-env-b2d02oya/overlay/lib/python3.8/site-packages/setuptools/__init__.py", line 153, in setup
        return distutils.core.setup(**attrs)
      File "/usr/local/lib/python3.8/distutils/core.py", line 108, in setup
        _setup_distribution = dist = klass(attrs)
      File "/tmp/pip-build-env-b2d02oya/overlay/lib/python3.8/site-packages/setuptools/dist.py", line 455, in __init__
        _Distribution.__init__(self, {
      File "/usr/local/lib/python3.8/distutils/dist.py", line 292, in __init__
        self.finalize_options()
      File "/tmp/pip-build-env-b2d02oya/overlay/lib/python3.8/site-packages/setuptools/dist.py", line 807, in finalize_options
        ep(self)
      File "/tmp/pip-build-env-b2d02oya/overlay/lib/python3.8/site-packages/setuptools_scm/integration.py", line 52, in infer_version
        config = Configuration.from_file(dist_name=dist_name)
      File "/tmp/pip-build-env-b2d02oya/overlay/lib/python3.8/site-packages/setuptools_scm/config.py", line 187, in from_file
        defn = _load_toml(data)
      File "/tmp/pip-build-env-b2d02oya/overlay/lib/python3.8/site-packages/setuptools_scm/config.py", line 59, in _lazy_tomli_load
        from tomli import loads
    ModuleNotFoundError: No module named 'tomli'
    ----------------------------------------
ERROR: Command errored out with exit status 1: /tmpfs/src/github/google-resumable-media-python/.nox/blacken/bin/python /tmpfs/src/github/google-resumable-media-python/.nox/blacken/lib/python3.8/site-packages/pip/_vendor/pep517/_in_process.py prepare_metadata_for_build_wheel /tmp/tmp42b1v5xx Check the logs for full command output.