pw_transfer: Add Bazel build file for Python unit test
pw_transfer: Add Bazel build file for Python unit test

This commit adds a Bazel build file for the pw_transfer Python
API, and the 'transfer_test' unit test.

This was primarily done to make it easier to add a Python client for
the pw_transfer integration test (in the next commit). The integration
test is exclusively built with Bazel and thus will require this build
file.

Bug: 670
Change-Id: Iec6cccf9d5bd717ad70bc07f86b53762039143ab
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/94000
Commit-Queue: Ted Pudlik <c7372cc3e451acc046049fca9e050bcb2a4a3d11@google.com>
Reviewed-by: Ted Pudlik <c7372cc3e451acc046049fca9e050bcb2a4a3d11@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed