Fix SyntaxWarning when comparing a literal with for identity in Python 3.8

[Morwenn]

Changelog: Fix: Fix SyntaxWarning when comparing a literal with for identity in Python 3.8
Docs: Omit

Since version 3.8 CPython emits a SyntaxWarning when one tries to compare a literal with is or is not (it is mentioned int eh changelog). I run into such a warning when installing the Conan from its develop branch on Windows:

d:\morwenn\projets\conan\conans\client\tools\win.py:283: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if len(version) is not 0:
d:\morwenn\projets\conan\conans\client\tools\win.py:293: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if len(property_) is not 0:

Like most SyntaxWarning it is likely to become an error in the future, so it is better to fix it sooner rather than later.

• Refer to the issue that supports this Pull Request: None
• If the issue has missing info, explain the purpose/use case/pain/need that covers this Pull Request.
• I've read the Contributing guide.
• I've followed the PEP8 style guides for Python code.
• I've opened another PR in the Conan docs repo to the develop branch, documenting this one.

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[claassistantio]

All committers have signed the CLA.

[memsharded]

Like most SyntaxWarning it is likely to become an error in the future, so it is better to fix it sooner rather than later.

Yes, agree. Thanks for submitting this fix, will merge it once CI finish, and it will be fixed in next Conan 1.21.