Python API: add get_input_topics()

[candlerb]

Fixes #5525

Motivation

Feature parity with Java and Go APIs

Modifications

Add get_input_topics() to Python Context/ContextImpl

Verifying this change

• This change is a trivial rework / code cleanup without any test coverage.

(There are no existing test cases for other ContextImpl methods like get_output_topic. But I have tested this locally within a pulsar function)

Does this pull request potentially affect one of the following parts:

• The public API: (python) adds new method

Documentation

• These methods missing from documentation anyway. PR Doc: add missing functions from Python context object #5524 adds get_output_topic; get_input_topics can be added once that has been merged (or if this one is merged first, then I'll update Doc: add missing functions from Python context object #5524) Added documentation under site2/

[candlerb]

Failed test is cpp, appears unrelated to this change.

[34/160] BasicEndToEndTest.testResendViaSendCallback (4234 ms)
�[0;33mNote: Google Test filter = BasicEndToEndTest.testResendViaSendCallback
�[m�[0;32m[==========] �[mRunning 1 test from 1 test case.
�[0;32m[----------] �[mGlobal test environment set-up.
�[0;32m[----------] �[m1 test from BasicEndToEndTest
�[0;32m[ RUN      ] �[mBasicEndToEndTest.testResendViaSendCallback
2019-10-31 18:43:54.603 INFO  ConnectionPool:85 | Created connection for pulsar://localhost:6650
2019-10-31 18:43:54.611 INFO  ClientConnection:324 | [127.0.0.1:37190 -> 127.0.0.1:6650] Connected to broker
2019-10-31 18:43:54.794 INFO  BatchMessageContainer:43 | { BatchContainer [size = 0] [batchSizeInBytes_ = 0] [maxAllowedMessageBatchSizeInBytes_ = 131072] [maxAllowedNumMessagesInBatch_ = 1000] [topicName = persistent://public/default/testResendViaListener] [producerName_ = ] [batchSizeInBytes_ = 0] [numberOfBatchesSent = 0] [averageBatchSize = 0]} BatchMessageContainer constructed
2019-10-31 18:43:54.795 INFO  HandlerBase:53 | [persistent://public/default/testResendViaListener, ] Getting connection from pool
2019-10-31 18:43:55.294 INFO  ProducerImpl:151 | [persistent://public/default/testResendViaListener, ] Created producer on broker [127.0.0.1:37190 -> 127.0.0.1:6650]
[34/160] BasicEndToEndTest.testResendViaSendCallback returned/aborted with exit code -11 (4234 ms)
...
FAILED TESTS (1/160):
    4234 ms: ./main BasicEndToEndTest.testResendViaSendCallback