Merge branch '1.3.x'

Merge branch '1.3.x'

* 1.3.x: (101 commits)
  rand.c: Show keyword argument as optional.
  mp.c: Fix punctuation error in docstrings.
  t/t-*.py: Use the `WriteBuffer.contents' property.
  t/t-bytes.py: Check that indexing, slicing, etc. return `C.ByteString'.
  t/t-algorithms.py: Add a simple test for `Keccak1600.copy'.
  t/t-algorithms.py: Add tests for other HSalsa20 and HChaCha key sizes.
  t/t-algorithms.py: Add AEAD tests.
  t/t-algorithms.py: Add tests for the new `KeySZ.pad' method.
  catacomb/__init__.py (KeySZRange.pad): Return correct value.
  algorithms.c: Propagate `AEADF_NOAAD' to `aad' objects.
  algorithms.c (AEADAAD.copy): Propagate the hashed length to the copy.
  t/: Add a test suite.
  ec.c: Don't lose error status when constructing points from a sequence.
  ec.c: Free partially constructed points coordinatewise.
  *.c: Be more careful about `PySequence_Size'.
  key.c: Reformat the rest of the `KeyError' constructor.
  key.c: Parse `KeyError' constructor arguments by hand.
  catacomb-python.h: Add a macro for raising `OverflowError'.
  key.c: Collect `KeyError' argument count as a separate step.
  key.c: Use tuple functions on `KeyError' argument tuple.
  ...