Make optional types explicit

[dbrattli]

Description

Make optional types explicit. A past version of PEP-484 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. For more info see: https://www.python.org/dev/peps/pep-0484/#id29

[rhuanbarreto]

@patkivikram can someone approve to run workflows in this PR?

[codecov-commenter]

Codecov Report

Merging #255 (9185026) into master (35d6885) will decrease coverage by 0.01%.
The diff coverage is 97.26%.

@@            Coverage Diff             @@
##           master     #255      +/-   ##
==========================================
- Coverage   94.25%   94.23%   -0.02%     
==========================================
  Files         100      100              
  Lines       10826    10829       +3     
  Branches     1479     1479              
==========================================
+ Hits        10204    10205       +1     
- Misses        551      553       +2     
  Partials       71       71              

Impacted Files	Coverage Δ
faust/agents/agent.py	98.06% <ø> (ø)
faust/app/base.py	99.51% <ø> (ø)
faust/auth.py	100.00% <ø> (ø)
faust/contrib/sentry.py	0.00% <ø> (ø)
faust/events.py	100.00% <ø> (ø)
faust/livecheck/app.py	100.00% <ø> (ø)
faust/models/base.py	100.00% <ø> (ø)
faust/models/record.py	96.38% <ø> (ø)
faust/sensors/datadog.py	98.71% <ø> (ø)
faust/sensors/monitor.py	99.64% <ø> (ø)
... and 36 more

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