Update dependencies, drop py35, test in py310, improve itemadapter performance

[elacuesta]

• Drop Python 3.5 support
• Add Python 3.10 support
• Bump minimum dependencies: Scrapy 1.6 (2019-01-30), python-scrapinghub 2.1.0 (2019-01-14)
• Improve performance by updating itemadapter usage
• Remove six as test dependency

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Calling itemadapter.ItemAdapter.is_item directly instead of the itemadapter.is_item alias (which imports itemadapter.ItemAdapter on each call) is faster:

from timeit import timeit
from itemadapter import is_item, ItemAdapter
from scrapy.item import Item

def test_dict_alias():
    return is_item({})

def test_dict_direct():
    return ItemAdapter.is_item({})

def test_item_alias():
    return is_item(Item())

def test_item_direct():
    return ItemAdapter.is_item(Item())

print("test_dict_alias: ", timeit(test_dict_alias, number=10**6))
print("test_dict_direct:", timeit(test_dict_direct, number=10**6))
print("test_item_alias: ", timeit(test_item_alias, number=10**6))
print("test_item_direct:", timeit(test_item_direct, number=10**6))

test_dict_alias:  2.2290923200000003
test_dict_direct: 1.314303395
test_item_alias:  3.5215694760000003
test_item_direct: 2.772791269

(At scrapy/itemadapter@1203b5e, with a 2020 Macbook Pro, Intel(R) Core(TM) i5-1038NG7 CPU @ 2.00GHz)

See also scrapy/itemadapter#59 and the issues/PRs linked there.

[codecov]

Codecov Report

Merging #66 (20967f1) into master (9c0c6f3) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 20967f1 differs from pull request most recent head 2c2ec56. Consider uploading reports for the commit 2c2ec56 to get more accurate results

@@            Coverage Diff             @@
##           master      #66      +/-   ##
==========================================
+ Coverage   95.04%   95.06%   +0.01%     
==========================================
  Files          14       14              
  Lines         747      750       +3     
==========================================
+ Hits          710      713       +3     
  Misses         37       37              

Impacted Files	Coverage Δ
sh_scrapy/extension.py	100.00% <100.00%> (ø)

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[elacuesta]

itemadapter does not support py35, so the tests fail. I think it's a good opportunity to just remove support here too.