allow for best practice code,

```
try:
  sc = SparkContext()
  app(sc)
finally:
  sc.stop()
```

to be written using a "with" statement,

```
with SparkContext() as sc:
  app(sc)
```

Author: Matthew Farrellee <matt@redhat.com>

Closes #2335 from mattf/SPARK-3458 and squashes the following commits:

5b4e37c [Matthew Farrellee] [SPARK-3458] enable python "with" statements for SparkContext