Apply PEP561

[phi-friday]

closes: #493
PEP561: https://peps.python.org/pep-0561/

Based on the previous discussion, type hinting using TypeVar was ruled out as it would be a barrier to entry for casual users. Instead, #507 was merged in.

In my opinion, this library fully meets the needs of the existing contributors and users,
so it makes sense to add py.typed to let users know that this library provides sufficient type hints.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.31%. Comparing base (aed6a25) to head (5069ca9).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #520      +/-   ##
==========================================
+ Coverage   96.19%   96.31%   +0.12%     
==========================================
  Files          10       11       +1     
  Lines         867      869       +2     
==========================================
+ Hits          834      837       +3     
+ Misses         33       32       -1     

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[till-m]

How do we check the effect of this change?

[till-m]

It seems this explicit include might not actually be necessary.

[phi-friday]

In local, I ran the poetry build command to verify that py.typed was included.
But I prefer to be explicit rather than implicit.
Of course, this is just my personal preference, but if you prefer simplicity, it seems better to remove it.
I'll fix it as soon as you point it out.

[phi-friday]

I fixed it