fix typing for optional arguments in interfaces

[vmeurisse]

Add missing Optional for method arguments.

This is the recommendation from pep-0484:

A past version of this PEP allowed type checkers to assume an optional type when the default value is None, as in this code:

def handle_employee(e: Employee = None): ...

This would have been treated as equivalent to:

def handle_employee(e: Optional[Employee] = None) -> None: ...

This is no longer the recommended behavior. Type checkers should move towards requiring the optional type to be made explicit.

[codecov-commenter]

Codecov Report

Merging #337 (3435c57) into master (d0a97fc) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #337   +/-   ##
=======================================
  Coverage   86.13%   86.13%           
=======================================
  Files          67       67           
  Lines        5879     5879           
  Branches     1218     1218           
=======================================
  Hits         5064     5064           
  Misses        596      596           
  Partials      219      219           

Impacted Files	Coverage Δ
apischema/conversions/converters.py	89.47% <ø> (ø)
apischema/conversions/utils.py	72.09% <ø> (ø)
apischema/deserialization/__init__.py	98.97% <ø> (ø)
apischema/graphql/resolvers.py	84.96% <ø> (ø)
apischema/graphql/schema.py	86.87% <ø> (ø)
apischema/json_schema/schema.py	85.59% <ø> (ø)
apischema/json_schema/types.py	92.68% <ø> (ø)
apischema/objects/conversions.py	78.00% <ø> (ø)
apischema/objects/getters.py	83.33% <ø> (ø)
apischema/serialization/__init__.py	84.96% <ø> (ø)
... and 8 more

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[wyfo]

Hi, thank you for your contribution!

Actually, I was aware of this deprecation, and I was planning to do it someday, but I kinda liked the old behavior, because it makes more obvious that the default value should not be used as an argument; there is indeed no interest to write deserialize(MyType, data, coerce=None) instead of deserialize(MyType, data).

That's why I took my time, but you're right, it's time to move on.