Add default format detection for Databook.load()

[nuno-andre]

Unlike Dataset.load(), which defaults to None, Databook.load() needs an explicit format value.

This code raises TypeError: load() missing 1 required positional argument: 'format':

with open('data.xlsx', 'rb') as fh:
    dbook = Databook().load(fh)

This PR also (without api changes):

• Adds some tests.
• Adds some exception messages.
• Fixes and updates core's docstrings and api docs.
• Simplifies Databook._package().
• LBYL → EAFP in .load().
• Adds an UnsupportedFormat exception for when detect_format fails to avoid the "Format 'None' cannot be loaded.'" message.

[codecov[bot]]

Codecov Report

Merging #488 (a875093) into master (7035d79) will increase coverage by 0.60%. The diff coverage is 89.28%.

@@            Coverage Diff             @@
##           master     #488      +/-   ##
==========================================
+ Coverage   90.67%   91.28%   +0.60%     
==========================================
  Files          28       28              
  Lines        2616     2638      +22     
==========================================
+ Hits         2372     2408      +36     
+ Misses        244      230      -14     

Impacted Files	Coverage Δ
src/tablib/core.py	87.05% <76.00%> (+3.08%)	:arrow_up:
tests/test_tablib.py	98.61% <100.00%> (+0.05%)	:arrow_up:

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[nuno-andre]

Not the best PR, indeed. It's a bunch of minor changes prior to push some adapters we're using in prod.

So, in order to ease these and other contributions, would you consider setting those conventions and adding a code formatter as a pre-commit hook? Are there guidelines for this at Jazzband?

[claudep]

Frankly, I think it would be better to let formatting issues aside and concentrate on "real" changes instead, separated by concern.

[nuno-andre]

Code formatting reduces diffs and merging conflicts, and saves developers' time. In what sense is not a "real" change?

[hugovk]

Thanks for the PR!

Formatting is a real change, but please create another PR or issue for that (or see https://github.com/jazzband/tablib/issues/401 and linked PRs) and keep this one focused. If anything, it'll help us get this format detection reviewed and merged sooner.

Thank you!

[nuno-andre]

Right, I should have searched for such an issue... I'm going to close this PR (it's already too unfocused) and I'll open separate PRs.

Anyway, are there some guidelines for linting and code formatting at Jazzband?

[hugovk]

It varies by project. This one has some linting done by pre-commit, which you run using the pre-commit commands or with tox -e lint. The CI will check it too.