fix: cleaner IPython import completion for Python 3.7+

[henryiii]

I’ve used Python 3.7 module-level __getattr__ for a while now to allow mistyping hist.axis import as hist.axes to show a warning but work anyway. However, I completely missed a fantastic usage for Python 3.7 module-level __dir__ : you can “fix” IPython’s tab completion to just show the items in __all__! Wow! One of the main things that bugged me about importing and defining unrelated things in modules, and one reason that boost-histogram has an _internal module. It was almost a parenthetical comment on the classic article here: https://snarky.ca/lazy-importing-in-python-3-7/

[sourcery-ai]

Sourcery Code Quality Report

✅  Merging this PR will increase code quality in the affected files by 2.12%.

Quality metrics	Before	After	Change
Complexity	10.61 🙂	10.21 🙂	-0.40 👍
Method Length	67.91 🙂	58.23 ⭐	-9.68 👍
Working memory	14.32 😞	13.72 😞	-0.60 👍
Quality	49.76% 😞	51.88% 🙂	2.12% 👍

Other metrics	Before	After	Change
Lines	713	722	9

Changed files	Quality Before	Quality After	Quality Change
src/hist/init.py	84.75% ⭐	89.99% ⭐	5.24% 👍
src/hist/axestuple.py	87.67% ⭐	88.97% ⭐	1.30% 👍
src/hist/plot.py	37.05% 😞	37.70% 😞	0.65% 👍
src/hist/tag.py	83.29% ⭐	83.29% ⭐	0.00%
src/hist/typing.py	78.72% ⭐	84.68% ⭐	5.96% 👍
src/hist/axis/init.py	79.74% ⭐	81.98% ⭐	2.24% 👍

Here are some functions in these files that still need a tune-up:

File	Function	Complexity	Length	Working Memory	Quality	Recommendation
src/hist/plot.py	plot_pull	23 😞	716 ⛔	22 ⛔	16.50% ⛔	Refactor to reduce nesting. Try splitting into smaller methods. Extract out complex expressions
src/hist/plot.py	plot2d_full	9 🙂	278 ⛔	14 😞	38.61% 😞	Try splitting into smaller methods. Extract out complex expressions
src/hist/plot.py	_curve_fit_wrapper	5 ⭐	156 😞	14 😞	50.72% 🙂	Try splitting into smaller methods. Extract out complex expressions
src/hist/plot.py	_expr_to_lambda	9 🙂	129 😞	11 😞	54.60% 🙂	Try splitting into smaller methods. Extract out complex expressions
src/hist/axis/init.py	Regular.__init__	2 ⭐	71 🙂	18 ⛔	60.35% 🙂	Extract out complex expressions

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