Add close() and context manager support to Client (#87)

[yashbudhia]

Fixes #87.

Client.__init__ creates a requests.Session() when none is passed in, but nothing ever closes it. In long-running processes, and in any test suite run with warnings enabled, every client that goes out of scope produces ResourceWarning: unclosed <socket.socket ...> as the pooled connections are garbage collected, which is the symptom reported in #87.

Change

• Client.close() closes the underlying session and releases its pooled connections.

• Client supports the with statement, so the session is closed on exit even if the block raises:

  with razorpay.Client(auth=(key_id, key_secret)) as client:
      client.payment.fetch(payment_id)

• A session passed in by the caller is closed by close() as well, matching the behaviour of requests.Session itself. This is documented in the README.

• Existing code that never closes the client is unaffected.

Tests

tests/test_client_session.py covers close(), the context manager return value, closing on normal exit, closing when the block raises, and the default session being closed. Full suite: 161 passed, 1 skipped.

Docs

README gets a short "close it when you are done" snippet under Usage, and CHANGELOG has an entry under Unreleased.