Add typings

[aecay]

• Do some dark metaclass magic to support python 3.6 (We can lose this
  if youʼre ok bumping the version dependency of this package to >=3.7,
  since from 3.7 typing.Mapping is a runtime synonym of
  collections.abc.Mapping).
• Bump the version of poetry. I got weird dependency issues when using
  poetry 1.0.0 to run the tests. Upgrading fixed these, so I didnʼt
  look into them too hard.
• Add tox.ini to support running the tests locally (gotta make sure 3.6
  still works 😉)
• Add mypy and tox to the poetry dev-dependencies

[codecov]

Codecov Report

Merging #45 (8a59355) into master (407caff) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 8a59355 differs from pull request most recent head c97c464. Consider uploading reports for the commit c97c464 to get more accurate results

@@            Coverage Diff            @@
##            master       #45   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           46        51    +5     
=========================================
+ Hits            46        51    +5     

Impacted Files	Coverage Δ
immutabledict/__init__.py	100.00% <100.00%> (ø)

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[corenting]

Thanks you for your contribution 👍 !
It looks good to me. Can you just squash all commits into the first the first one and sign it ?
I will try to integrate this into a release ASAP to fix #41 .

[aecay]

Done 🙂 thanks

[corenting]

Merged :) Thanks again for your PR !