use type hints in APIs users will call: downloading, show type hint in documentation

[lisphilar]

Related issues

#1258

What was changed

• use type hints in APIs users will call: downloading
• show type hints in documentation using sphinx_autodoc_typehints and sphinx.ext.intersphinx

[sourcery-ai]

Sourcery Code Quality Report

❌  Merging this PR will decrease code quality in the affected files by 0.83%.

Quality metrics	Before	After	Change
Complexity	1.40 ⭐	1.36 ⭐	-0.04 👍
Method Length	85.40 🙂	92.60 🙂	7.20 👎
Working memory	5.45 ⭐	5.23 ⭐	-0.22 👍
Quality	76.44% ⭐	75.61% ⭐	-0.83% 👎

Other metrics	Before	After	Change
Lines	190	200	10

Changed files	Quality Before	Quality After	Quality Change
covsirphy/downloading/downloader.py	68.01% 🙂	67.81% 🙂	-0.20% 👎
docs/conf.py	84.86% ⭐	82.10% ⭐	-2.76% 👎

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

File	Function	Complexity	Length	Working Memory	Quality	Recommendation
covsirphy/downloading/downloader.py	DataDownloader.layer	4 ⭐	166 😞	9 🙂	59.39% 🙂	Try splitting into smaller methods

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• 🙂 good
• 😞 poor
• ⛔ very poor

The 👍 and 👎 indicate whether the quality has improved or gotten worse with this pull request.

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