title: Pyenv won't install python 3 on Mac os 11
body: <p>Trying to install Python 3.9.0 on Mac OS 11.0.1 with
  <code>pyenv install 3.9.0</code>.</p>
  <p>Results:</p>
  <pre><code>python-build: use readline from homebrew
  Downloading Python-3.9.0.tar.xz...
  -&gt; https://www.python.org/ftp/python/3.9.0/Python-3.9.0.tar.xz
  Installing Python-3.9.0...
  python-build: use readline from homebrew
  python-build: use zlib from xcode sdk
  
  BUILD FAILED (OS X 11.0.1 using python-build 20180424)
  
  Inspect or clean up the working tree at /var/folders/n6/q2b78971589bltfczw539flh0000gn/T/python-build.20201114175722.7103
  Results logged to /var/folders/n6/q2b78971589bltfczw539flh0000gn/T/python-build.20201114175722.7103.log
  
  Last 10 log lines:
  checking for python3... python3
  checking for --enable-universalsdk... no
  checking for --with-universal-archs... no
  checking MACHDEP... &quot;darwin&quot;
  checking for gcc... clang
  checking whether the C compiler works... no
  configure: error: in `/var/folders/n6/q2b78971589bltfczw539flh0000gn/T/python-build.20201114175722.7103/Python-3.9.0':
  configure: error: C compiler cannot create executables
  See `config.log' for more details
  make: *** No targets specified and no makefile found.  Stop.
  </code></pre>
  <p>Many suggestions in similar questions seem to revolve around XCode, but <code>xcode-select --install</code> says command line tools already installed.</p>
  <p>Any help appreciated.</p>