Hyperframe

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**Backwards Incompatible API Changes**

- Parsing padded frames that have invalid padding sizes now throws an
``InvalidPaddingError``.

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**API Changes**

- When an unknown frame is encountered, ``parse_frame_header`` now throws a
``ValueError`` subclass: ``UnknownFrameError``. This subclass contains the
frame type and the length of the frame body.

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**API Changes**

- Frames parsed from binary data now carry a ``body_len`` attribute that
matches the frame length (minus the frame header).

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**API Changes**

- Attempting to parse unrecognised frames now throws ``ValueError`` instead of
``KeyError``. Thanks to Kriechi!
- Flags are now validated for correctness, preventing setting flags that
``hyperframe`` does not recognise and that would not serialize. Thanks to
mhils!
- Frame properties can now be initialized in the constructors. Thanks to mhils
and Kriechi!
- Frames that cannot be sent on a stream now have their stream ID defaulted
to ``0``. Thanks to Kriechi!

**Other Changes**

- Frames have a more useful repr. Thanks to mhils!

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- Fix a bug where ``FRAME_MAX_LEN`` was one byte too small.

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- Add ``body_len`` property to frames to enable introspection of the actual
frame length. Thanks to jdecuyper!