Pyiso8583

------------------
- Add support for binary field length
- Drop support for Python 3.6
- Migration from 2.x.x:
- 2.x.x used `len_enc` set to `b` and `bcd` to represent BCD length encoding.
- 3.x.x changed that where `len_enc` set to `b` represents binary length encoding.
- To migrate from 2.x.x update `len_enc` in all specificatations from `b` to `bcd`.

------------------
- Provide friendlier error messages when failing to encode/decode field, field length, and bitmap.
- Clarify Binary-coded decimal field length configuration. Added ``bcd`` value to ``len_enc``
which is the same as the existing ``b`` value. Both mean that the length is to be encoded as BCD.

------------------
- Added support for fields measured in nibbles (half-bytes).

------------------
- Fixed issue `4 <https://github.com/knovichikhin/pyiso8583/issues/4>`_. Encode secondary bitmap in upper case.

------------------
- Include inline type information into the distribution according to `PEP 561 <https://www.python.org/dev/peps/pep-0561/>`_.
- Address remaining type hint issues.

------------------
**Backwards incompatible**:
- :func:`iso8583.decode` is updated not to produce bitmap key ``'bm'``.
- :func:`iso8583.encode` is updated not to expect bitmap key ``'bm'`` to define
fields to encode. The decision on what fields to encode is based on
numeric fields in the range of ``'1'-'128'`` present in the decoded
dictionary.
- :func:`iso8583.add_field` and :func:`iso8583.del_field` are removed. With the
removal of bitmap set ``'bm'`` default Python dictionary methods are
enough.
- :func:`iso8583.encode` now removes secondary bitmap key ``'1'`` from the decoded
dictionary if no ``'65'-'128'`` fields are present.
- :func:`iso8583.pp` function signature changed. The first parameter is renamed
from ``doc_dec`` to ``doc``.
Other changes:
- :func:`iso8583.pp` handles both encoded and decoded dictionary output.
- :func:`iso8583.pp` handles output folding. The defaul line width is set to 80.
Line width can be configured using new ``line_width`` parameter.