py-sniffio: update to 1.2.0, enable for Python 3.9

[herbygillot]

Verification

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• followed our Commit Message Guidelines?
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• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL?
• checked your Portfile with port lint?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?

[macportsbot]

Notifying maintainers:
@jandemter for port py-sniffio.

[macportsbot]

Travis Build #14773 Passed.

Lint results

--->  Verifying Portfile for py-sniffio
--->  0 errors and 0 warnings found.

Port py38-sniffio success on xcode10.3. Log
Port py-sniffio success on xcode10.3. Log
Port py36-sniffio success on xcode10.3. Log
Port py37-sniffio success on xcode10.3. Log
Port py39-sniffio success on xcode10.3. Log
Port py38-sniffio success on xcode9.4. Log
Port py-sniffio success on xcode9.4. Log
Port py36-sniffio success on xcode9.4. Log
Port py37-sniffio success on xcode9.4. Log
Port py39-sniffio success on xcode9.4. Log
Port py38-sniffio success on xcode8.3. Log
Port py-sniffio success on xcode8.3. Log
Port py36-sniffio success on xcode8.3. Log
Port py37-sniffio success on xcode8.3. Log
Port py39-sniffio success on xcode8.3. Log
Port py38-sniffio success on xcode7.3. Log
Port py-sniffio success on xcode7.3. Log
Port py36-sniffio success on xcode7.3. Log
Port py37-sniffio success on xcode7.3. Log