_key and _type

[manycoding]

This pr makes schema validation a bit slower, but it's constant and I think not huge.
before, 50k items a.glance()
335 ms ± 39.3 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
this pr
350 ms ± 19.8 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

Some incentives are given in #104 . The main thing is that we keep Spidermon and Arche schemas more consistent by dropping these keys, and it's 2 fields less to describe in a schema.
Also it kind of nicer since we don't have to include _key in pandas selectors anymore, index is always there.

Now df looks like:

One thing to note is that raw is modified after the first pass.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (0.3.6dev@bc42578). Click here to learn what that means.
The diff coverage is 88.63%.

@@             Coverage Diff             @@
##             0.3.6dev     #106   +/-   ##
===========================================
  Coverage            ?   78.97%           
===========================================
  Files               ?       23           
  Lines               ?     1598           
  Branches            ?      277           
===========================================
  Hits                ?     1262           
  Misses              ?      290           
  Partials            ?       46

Impacted Files	Coverage Δ
src/arche/data_quality_report.py	78.72% <ø> (ø)
src/arche/tools/json_schema_validator.py	96.07% <100%> (ø)
src/arche/rules/others.py	100% <100%> (ø)
src/arche/rules/duplicates.py	100% <100%> (ø)
src/arche/tools/schema.py	78% <100%> (ø)
src/arche/readers/items.py	85.24% <100%> (ø)
src/arche/rules/json_schema.py	100% <100%> (ø)
src/arche/report.py	97.72% <100%> (ø)
src/arche/rules/price.py	58% <50%> (ø)
src/arche/arche.py	85.21% <66.66%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update bc42578...f52bca1. Read the comment docs.

[raphapassini]

LGTM.
One thing is that looking at the tests seems that we are always append the SH_URL to the item index, this is True? If so, I think can be a good idea to have this parametrized somehow for people who want to run Arche unrelated to Scrapy Cloud items.

[manycoding]

One thing is that looking at the tests seems that we are always append the SH_URL to the item index, this is True?

No, index depends on how a user gets the data. If from Cloud, url will be there, if by other means - it's up to a user to create his own index.