Fix PEP 561 typed package

[DeviousStoat]

As specified by PEP 561, the py.typed file needs to be included in the python package itself.

In this case, if I install the package from the master branch of the project, a static type checker like pyright doesn't find the py.typed file and no type is used.

To reproduce:

$ pip install git+https://github.com/scrapy/parsel.git@master

# t.py
from parsel import Selector

element = Selector("").css("#some_id").get()
reveal_type(element)

$ pyright t.py
...
type of "element" is "Unknown"
...

Expected should be: type of "element" is "str | None"

[codecov[bot]]

Codecov Report

Merging #240 (0c32437) into master (f5f73d3) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #240   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         5           
  Lines          290       290           
  Branches        59        59           
=========================================
  Hits           290       290           

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[DeviousStoat]

Thank you! I see black failed on a file I didn't change, I am not sure why, should I do something about it in this PR?

[wRAR]

I suspect it's a change in the black's behavior since we last ran it. You should ignore it.