Add mypy checks

[IKarbowiak]

Add mypy to CI and pre-commit checks. Fix mypy errors.
Closes #4720

Pull Request Checklist

1. Privileged views and APIs are guarded by proper permission checks.
2. All visible strings are translated with proper context.
3. All data-formatting is locale-aware (dates, numbers, and so on).
4. Database queries are optimized and the number of queries is constant.
5. Database migration files are up to date.
6. The changes are tested.
7. GraphQL schema and type definitions are up to date.
8. Changes are mentioned in the changelog.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@7964037). Click here to learn what that means.
The diff coverage is 97.5%.

@@           Coverage Diff            @@
##             master   #5150   +/-   ##
========================================
  Coverage          ?   91.7%           
========================================
  Files             ?     260           
  Lines             ?   16838           
  Branches          ?    1482           
========================================
  Hits              ?   15442           
  Misses            ?    1008           
  Partials          ?     388

Impacted Files	Coverage Δ
saleor/graphql/order/schema.py	100% <ø> (ø)
saleor/graphql/utils.py	89.89% <ø> (ø)
saleor/graphql/product/types/attributes.py	98.01% <ø> (ø)
saleor/graphql/menu/types.py	89.74% <ø> (ø)
saleor/graphql/checkout/schema.py	100% <ø> (ø)
saleor/graphql/account/mutations/staff.py	97.02% <ø> (ø)
saleor/product/utils/availability.py	97.61% <ø> (ø)
saleor/core/middleware.py	52.5% <ø> (ø)
saleor/graphql/account/mutations/base.py	98.66% <ø> (ø)
saleor/graphql/menu/schema.py	100% <ø> (ø)
... and 26 more

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[django-queries]

Here is the report for 64541ac (mirumee/saleor @ add-mypy-checks)
Base comparison is 9ac4ee9.

No differences were found. (click me)

# api.benchmark checkout
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  add billing address to checkout            	         34	         34	             20
  add shipping to checkout                   	          7	          7	              0
  checkout payment charge                    	         10	         10	              0
  complete checkout                          	          8	          8	              0
  create checkout                            	         50	         50	             24

# api.benchmark homepage
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  retrieve main menu                         	          5	          5	              0
  retrieve product list                      	          4	          4	              0
  retrieve secondary menu                    	          5	          5	              0
  retrieve shop                              	          2	          2	              0

# api.benchmark product
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  product details                            	         14	         14	              2
  retrieve product attributes                	          9	          9	              0

# api.benchmark variant
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  product variant bulk create                	         51	         51	              3
  retrieve variant list                      	         16	         16	              6

# api product sorting attributes
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  sort product not having attribute data     	         21	         21	              0

[IKarbowiak]

Some clarification:

1. I was trying to avoid # type: ignore annotation, but it wasn't possible in all places. For example mypy doesn't take into account hasattr. There are tickets about that on their github so maybe it'll be resolved soon. Type inference and hasattr python/mypy#1424 Take into account the results of hasattr()/dir(), similarly to isinstance and so on… python/mypy#4944
2. Instead of just # type: ignore I wanted to specified code error like: # type: ignore[attr-defined] but the flake8 has problem with that and I can't find solution.
   

[maarcingebala]

I guess it's fine to have those # type: ignore occurrences in a few places if we can't do anything about it.

[krzysztofwolski]

💪 lots of improvements!