bpo-30731: python.manifest fix

[segevfiner]

I'm not seeing the typo mentioned in the issue (PC/python.manifest:3) but the xmlns is missing so I added it. Also trimmed some trailing whitespace because really...

For reference, here is a manifest generated by default on stuff built with VS2015:

<?xml version='1.0' encoding='UTF-8' standalone='yes'?>
<assembly xmlns='urn:schemas-microsoft-com:asm.v1' manifestVersion='1.0'>
  <trustInfo xmlns="urn:schemas-microsoft-com:asm.v3">
    <security>
      <requestedPrivileges>
        <requestedExecutionLevel level='asInvoker' uiAccess='false' />
      </requestedPrivileges>
    </security>
  </trustInfo>
</assembly>

Documentation on this seems surprisingly poor on MSDN. Or maybe I just failed to find it...

[mention-bot]

@segevfiner, thanks for your PR! By analyzing the history of the files in this pull request, we identified @zooba to be a potential reviewer.

[zooba]

I want to merge this, but now it has a bug it needs a NEWS.d entry. Can you add one?

[zooba]

Thanks!

[AraHaan]

Sorry to necro this PR but I feel like having comments for the versions of windows on each entry on the compatibility section is needed and so I will do a followup PR to add it soon.