Fix the only non-C90 comment to be C90 compatible.

[ndparker]

Including pyport.h into a module compiled with a C89/90 compiler gives the error:

C++ style comments are not allowed in ISO C90.

It would be nice to be included into the next python 3.6 version as well.

[the-knights-who-say-ni]

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[mention-bot]

@ndparker, thanks for your PR! By analyzing the history of the files in this pull request, we identified @benjaminp, @tim-one, @Yhg1s, @mdickinson and @mhammond to be potential reviewers.

[serhiy-storchaka]

Could you please backport this change to 3.6 @ndparker?

[ndparker]

FWIW, I've signed the CLA now :-)

@serhiy-storchaka Thanks for merging. Should I open another PR for python 3.6 or how is it supposed to be done?

[serhiy-storchaka]

Thanks @ndparker. Signing the CLA for such trivial change is not needed, but i hope this is not your last contribution.

Just open another PR for Python 3.6.