Python API: PTransform should be immutable and reusable

While the Java API seems fine the Python API is (at least) counterintuitive.

Let's see the following example:
```

p1 = beam.Pipeline()
p2 = beam.Pipeline()
node = 'ReadTrainData' >> beam.io.ReadFromText("/tmp/aaa.txt")
p1
| node
p2 | node //fails here
```

The code above will fail because the _node_ somehow remembers that it was already attached to _p1_. In fact, unlike in Java, the | (apply) method is defined on the _PTransform_.

If any, only the pipeline object should be mutable here.

Imported from Jira [BEAM-8140](https://issues.apache.org/jira/browse/BEAM-8140). Original Jira may contain additional context.
Reported by: chris_suchanek.