chore: Use typing.TYPE_CHECKING instead of MYPY

[rapsealk]

Hello graphene!

I found that the code base is using MYPY as a constant to tell whether a type checker is running.

According to the mypy docs, however, mypy introduces a more commonly used constant: typing.TYPE_CHECKING. So I'd like to suggest using this rather than MYPY.

As typing.TYPE_CHECKING has been added since Python 3.5.2 and graphene supports from Python 3.6, there would be no problem with backporting.

Reference:

• https://docs.python.org/ko/3/library/typing.html#typing.TYPE_CHECKING

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (c636d98) 96.01% compared to head (9b41db2) 96.01%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1503   +/-   ##
=======================================
  Coverage   96.01%   96.01%           
=======================================
  Files          51       51           
  Lines        1755     1755           
=======================================
  Hits         1685     1685           
  Misses         70       70           

Impacted Files	Coverage Δ
graphene/types/inputobjecttype.py	97.50% <100.00%> (ø)
graphene/types/interface.py	93.33% <100.00%> (ø)
graphene/types/mutation.py	92.45% <100.00%> (ø)
graphene/types/objecttype.py	93.61% <100.00%> (ø)
graphene/types/union.py	91.30% <100.00%> (ø)

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[erikwrede]

Thanks! 😊