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I'm closing this as a third-party issue with older versions of PowerShell. Newer versions of PowerShell set the output encoding to UTF-8 without a BOM preamble. For example:
 PS C:\> $PSVersionTable.PSVersion
 Major Minor Patch PreReleaseLabel BuildLabel
 ----- ----- ----- --------------- ----------
 7 0 3
 PS C:\> $OutputEncoding.EncodingName
 Unicode (UTF-8)
 PS C:\> echo ¡¢£¤\ | py -3 -X utf8 -c "print(ascii(input()))"
 '\xa1\xa2\xa3\xa4\xa5'
It's still possible to manually set the output encoding to include a BOM preamble. For example:
 PS C:\> $OutputEncoding = [System.Text.Encoding]::UTF8
 PS C:\> $OutputEncoding.GetPreamble()
 239
 187
 191
 PS C:\> echo ¡¢£¤\ | py -3 -X utf8 -c "print(ascii(input()))"
 '\ufeff\xa1\xa2\xa3\xa4\xa5'
I don't know what would be appropriate for Python's I/O stack in terms of detecting and handling a UTF-8 preamble on any type of file (console/terminal, pipe, disk), i.e. using the "utf-8-sig" encoding instead of "utf-8", as opposed to just letting scripts detect and handle an initial BOM character (U+FEFF) however they see fit. But that discussion needs a new issue if people are interested in supporting new behavior.