Add type stubs

[sethmlarson]

Saw that there were no type stubs for idna so went ahead and put them together. If you drop Python 3.4 (<1% of downloads) I can change them to be inline instead of .pyi files. Let me know what you think!

[sethmlarson]

@kjd You've approved the PR but not merged, is there something still left to do here? Also what are your thoughts on dropping Python 3.4?

[kjd]

I didn't merge the commit before reconciling that very question, since the PR removes the 3.4 trove classifier.

As to whether to drop 3.4, I'd like to do an inventory on the relying parties on this library to see where their Python support lies. I know requests is one notable user, and they report supporting 3.5+, but a broader sampling would be useful to make sure such a move wouldn't unexpectedly impact significant packages.

[sethmlarson]

Got it! I can gather a list of supported Python versions for dependent packages and report back.

[sethmlarson]

@kjd Here's direct dependency info I grabbed from my PyPI dependency dataset:

anyio python (>=3.6.2) idna (>=2.8)
aws-service-catalog-puppet python (>=3.7) idna (==2.8)
dbt-core python (>=3.6.3) idna (<2.10)
dkcloudcommand python (>=3.6) idna (==2.6)
email-validator python () idna (>=2.0.0)
flexget python (>=3.6) idna (==2.8)
lftools python () idna (==2.9)
moto python () idna (<3,>=2.5)
pl-nightly python (>=3.6,<3.8) idna (>=2.10), idna (>=2.9)
requests python (>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*) idna (<3,>=2.5)
seleniumbase python (>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*) idna (==2.10)
sickrage python (>=3) idna (==2.10)
snowflake-connector-python python (>=3.6) idna (<3,>=2.5)
tldextract python (>=3.5) 
trio python (>=3.6) 
urlextract python () 
yarl python (>=3.6) idna (>=2.0)

so the only dependencies that are effected would be ones that unpin idna 3.* and allow Python 3.4, those are:

email-validator (allows any Python)
sickrage (allows Python>3)
urlextract (allows any Python and idna version)

[kjd]

Thanks for the quick and comprehensive answer. I guess I have no problem targeting 3.5+ moving forward.