Fedora 30: ImportError: libnsl.so.1 when importing apache_beam in Python 3

`When importing apache_beam in python, it fails because it can't find libnsl.so.1.`

It is fixed by running 'dnf install libnsl'.  This appears to be a dependency of Apache Arrow.

`output:`

`Singularity tfx:~/tfx_image_example_gen> python`
`Python 3.6.9 |Anaconda, Inc.| (default, Jul 30 2019, 19:07:31) `
`[GCC 7.3.0] on linux`
`Type "help", "copyright", "credits" or "license" for more information.`
`>>> import apache_beam`
`/opt/snps/envs/tfx/lib/python3.6/site-packages/apache_beam/__init__.py:84: UserWarning: Some syntactic constructs of Python 3 are not yet fully supported by Apache Beam.`
` 'Some syntactic constructs of Python 3 are not yet fully supported by '`
`Traceback (most recent call last):`
` File "", line 1, in `
` File "/opt/snps/envs/tfx/lib/python3.6/site-packages/apache_beam/__init__.py", line 98, in `
` from apache_beam import io`
` File "/opt/snps/envs/tfx/lib/python3.6/site-packages/apache_beam/io/__init__.py", line 29, in `
` from apache_beam.io.parquetio import *`
` File "/opt/snps/envs/tfx/lib/python3.6/site-packages/apache_beam/io/parquetio.py", line 45, in `
` import pyarrow as pa`
` File "/opt/snps/envs/tfx/lib/python3.6/site-packages/pyarrow/__init__.py", line 49, in `
` from pyarrow.lib import cpu_count, set_cpu_count`
`ImportError: libnsl.so.1: cannot open shared object file: No such file or directory`

Imported from Jira [BEAM-8110](https://issues.apache.org/jira/browse/BEAM-8110). Original Jira may contain additional context.
Reported by: robert.lugg.