Bump pyarrow from 0.17.1 to 4.0.0 in /megalist_dataflow

Bumps [pyarrow](https://github.com/apache/arrow) from 0.17.1 to 4.0.0.
- [Release notes](https://github.com/apache/arrow/releases)
- [Commits](apache/arrow@apache-arrow-0.17.1...apache-arrow-4.0.0)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

megalist_dataflow/requirements.txt

@@ -14,7 +14,7 @@ pytest-mock==3.2.0
 requests-mock==1.8.0
 pytz==2020.1
 wheel==0.34.2
-pyarrow==0.17.1
+pyarrow==4.0.0
 aiohttp==3.6.2
 bloom-filter==1.3
 six==1.13.0