title: Problems installing python 3.6 with pyenv on Mac OS Big Sur
body: <p>When installing Python 3.6.13 on my MacBook Air, OS Big Sur, using the command pyenv install 3.6.13 I get this error:</p>
  <pre><code>Installing Python-3.6.13...
  python-build: use readline from homebrew
  python-build: use zlib from xcode sdk
  
  BUILD FAILED (OS X 11.3 using python-build 20180424)
  
  Inspect or clean up the working tree at /var/folders/7j/0qtpb8vs1_s34ynv0f6rrs840000gn/T/python-build.20210304114832.65954
  Results logged to /var/folders/7j/0qtpb8vs1_s34ynv0f6rrs840000gn/T/python-build.20210304114832.65954.log
  
  Last 10 log lines:
  ./Modules/posixmodule.c:8210:15: error: implicit declaration of function 'sendfile' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
          ret = sendfile(in, out, offset, &amp;sbytes, &amp;sf, flags);
                ^
  ./Modules/posixmodule.c:10432:5: warning: code will never be executed [-Wunreachable-code]
      Py_FatalError(&quot;abort() called from Python code didn't abort!&quot;);
      ^~~~~~~~~~~~~
  1 warning and 1 error generated.
  1 warning generated.
  make: *** [Modules/posixmodule.o] Error 1
  make: *** Waiting for unfinished jobs....
  </code></pre>
  <p>Any suggestions on how to fix this or another work around to install python 3.6.13 would be greatly appreciated. I currently have python 3.8, but need 3.6 in order to run:</p>
  <pre><code>conda install -c deezer-research spleeter 
  </code></pre>