Display blank lines at half-height in fancyvrb Verbatim environment

I’m typesetting code that includes lots of blank lines, like this:

documentclass{minimal}
usepackage{fancyvrb}

begin{document}
begin{Verbatim}
def xyz():
    """Some documentation.

    More docs.

    More docs."""

    some code

    more code
end{Verbatim}
end{document}

For compactness, I’d like to shrink the blank lines (left is before, right is after, red indicates blank lines):

Note that I don’t want to change the normal line spacing (the def in the example above doesn’t get closer to the """ right below it).

I need fancyvrb as the contents are actually generated using pygments. I tried reading through the definition of Verbatim using latexdef. It seems that it makes ^^M active and that’s how it preserves newlines. I imagine I could redefine that to count how many newlines we’ve seen in a row, but I’m not sure how to check whether they were immediately consecutive — maybe an if@nextchar?

As a first step, I tried adding markers manually to shrink the lines:

documentclass{minimal}
usepackage{fancyvrb}

begin{document}
begin{Verbatim}[commandchars={}]
def xyz():
    """Some documentation.
unskipvspace{-0.5baselineskip}
    More docs.
unskipvspace{-0.5baselineskip}
    More docs.
    """
unskipvspace{-0.5baselineskip}
    some code
unskipvspace{-0.5baselineskip}
    more code
end{Verbatim}
end{document}

But I got an error, and I can’t tell where it comes from:

ERROR: Missing number, treated as zero.

--- TeX said ---
<to be read again> 
                   unhbox 
l.8 unskipvspace{-0.5baselineskip}

How can I reduce the height of blank lines in Verbatim environments?

documentclass{article}
usepackage{fancyvrb}

makeatletter
newifif@FV@emptyline

begingroup
catcode`^^M=active%
gdefFancyVerbGetLine#1^^M{%
  @nil%
  FV@CheckEnd{#1}%
  %% modification begin
  if@FV@emptyline
    vskip-.5baselineskip
  fi
  %% modification end
  ifx@tempaFV@EnvironName%            % True if end is found
    ifx@tempbFV@@@CheckEndelseFV@BadEndErrorfi%
    letnextFV@EndScanning%
  else%
    defFV@Line{#1}%
    %% modification begin
    ifxFV@Line@empty
      global@FV@emptylinetrue
    else
      global@FV@emptylinefalse
    fi
    %% modification end
    defnext{FV@PreProcessLineFV@GetLine}%
  fi%
  next}%
endgroup
makeatother

begin{document}

begin{Verbatim}[numberblanklines=false,numbers=left]
def xyz():
    """Some documentation.

    More docs.

    More docs.
    """

    some code

    more code
end{Verbatim}

end{document}