Liac-arff

This is the last release supporting Python 2.

* maintenance: drop support for Python 3.3 and 3.4, add support for Python 3.8
* maintenance: improve compatibility with the upcoming Python 3.10
* maintenance: automatically build the documentation.
* maintenance: fix mypy errors

* enhancement: load data progressively with generator `return_type`.
* enhancement: standard Java escape sequences are now decoded in string
attributes, and non-printable characters are now encoded with escaping.
* fix: match all possible separator spaces to add quotes when encoding into
ARFF. These separator spaces will be preserved when decoding the ARFF files.

* maintenance: replace two bare ``raise`` by appropriate ``raise Exception``
statements
* maintenance: avoid deprecation warning in Python >= 3.6

- enhancement: improvements to loading runtime (issue 76)
- fix: several bugs in decoding and encoding quoted and escaped values,
particularly in loading sparse ARFF.
- fix 52: Circumvent a known bug when loading sparse data written by WEKA

- new: test for python3.7 and pypy3

- fix: better support for string and nominal features containing escape
characters (issue 69).