chore(deps): update dependency pyarrow to v4 (#185)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [pyarrow](https://arrow.apache.org/) | `==3.0.0` -> `==4.0.0` | [![age](https://badges.renovateapi.com/packages/pypi/pyarrow/4.0.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/pypi/pyarrow/4.0.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/pypi/pyarrow/4.0.0/compatibility-slim/3.0.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/pypi/pyarrow/4.0.0/confidence-slim/3.0.0)](https://docs.renovatebot.com/merge-confidence/) |

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packages/google-cloud-bigquery-storage/samples/to_dataframe/requirements.txt

@@ -1,7 +1,7 @@
 google-auth==1.28.1
 google-cloud-bigquery-storage==2.4.0
 google-cloud-bigquery==2.13.1
-pyarrow==3.0.0
+pyarrow==4.0.0
 ipython==7.10.2; python_version > '3.0'
 ipython==5.9.0; python_version < '3.0'
 pandas==0.25.3; python_version > '3.0'