In case of a partial write BufferedIOBase doesn't return the length of data written, but gives a BlockingIOError that contains the number of characters written.
See https://docs.python.org/3/library/io.html#io.BufferedIOBase.write

Also, RawIOBase returns None (and not 0) when no data can be written.

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This uses KeyTypeManagers to implement the ECIES Key Managers; making the code slightly simpler.

The main simplification happens when we update the tests in a follow up cl.

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We will migrate the test in an upcoming cl.

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We will migrate the test in an upcoming cl.

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We plan to remove these; so we instead manually register all key managers in python.

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To replicate:
find -name BUILD.bazel -exec sed -Ei 's/\s+# Apache 2.0$//' {} \;
find -name BUILD.bazel -exec sed -Ei 's/\s+# Apache 2.0 # keep$/  # keep/' {} \;

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