Update instruction

[ah1102]

Update the brew install python statement installed by python3. Python2 is assigned to EOL. The pip install Cheetah command no longer works, and pip3 install Cheetah results in an error.

brew cask install gqrx

Apple@iMac-Apple ~ % brew cask install gqrx
Updating Homebrew...
==> Auto-updated Homebrew!
Updated 2 taps (homebrew/core and homebrew/cask).
==> Updated Formulae
fastlane                                 v8
==> Updated Casks
homebrew/cask/cctalk

==> Downloading https://github.com/csete/gqrx/releases/download/v2.11.5/Gqrx-2.1
Already downloaded: /Users/Apple/Library/Caches/Homebrew/downloads/21e7343a967187ddb56723d54f5f726bf378704c3486520fb34618ba61a4032d--Gqrx-2.11.5.dmg
==> Verifying SHA-256 checksum for Cask 'gqrx'.
==> Installing Cask gqrx
==> Moving App 'Gqrx.app' to '/Applications/Gqrx.app'.
==> Linking Binary 'airspy_info' to '/usr/local/bin/airspy_info'.
==> Linking Binary 'airspy_spiflash' to '/usr/local/bin/airspy_spiflash'.
==> Linking Binary 'hackrf_cpldjtag' to '/usr/local/bin/hackrf_cpldjtag'.
==> Linking Binary 'hackrf_debug' to '/usr/local/bin/hackrf_debug'.
==> Linking Binary 'hackrf_info' to '/usr/local/bin/hackrf_info'.
==> Linking Binary 'hackrf_spiflash' to '/usr/local/bin/hackrf_spiflash'.
==> Linking Binary 'hackrf_transfer' to '/usr/local/bin/hackrf_transfer'.
==> Linking Binary 'rtl_adsb' to '/usr/local/bin/rtl_adsb'.
==> Linking Binary 'rtl_eeprom' to '/usr/local/bin/rtl_eeprom'.
==> Linking Binary 'rtl_fm' to '/usr/local/bin/rtl_fm'.
==> Linking Binary 'rtl_power' to '/usr/local/bin/rtl_power'.
==> Linking Binary 'rtl_sdr' to '/usr/local/bin/rtl_sdr'.
==> Linking Binary 'rtl_tcp' to '/usr/local/bin/rtl_tcp'.
==> Linking Binary 'rtl_test' to '/usr/local/bin/rtl_test'.
==> Linking Binary 'SoapySDRUtil' to '/usr/local/bin/soapysdrutil'.
==> Linking Binary 'volk_profile' to '/usr/local/bin/volk_profile'.
==> Linking Binary 'gqrx.wrapper.sh' to '/usr/local/bin/gqrx'.
==> Unlinking Binary '/usr/local/bin/volk_profile'.
==> Unlinking Binary '/usr/local/bin/soapysdrutil'.
==> Unlinking Binary '/usr/local/bin/rtl_test'.
==> Unlinking Binary '/usr/local/bin/rtl_tcp'.
==> Unlinking Binary '/usr/local/bin/rtl_sdr'.
==> Unlinking Binary '/usr/local/bin/rtl_power'.
==> Unlinking Binary '/usr/local/bin/rtl_fm'.
==> Unlinking Binary '/usr/local/bin/rtl_eeprom'.
==> Unlinking Binary '/usr/local/bin/rtl_adsb'.
==> Unlinking Binary '/usr/local/bin/hackrf_transfer'.
==> Unlinking Binary '/usr/local/bin/hackrf_spiflash'.
==> Unlinking Binary '/usr/local/bin/hackrf_info'.
==> Unlinking Binary '/usr/local/bin/hackrf_debug'.
==> Unlinking Binary '/usr/local/bin/hackrf_cpldjtag'.
==> Unlinking Binary '/usr/local/bin/airspy_spiflash'.
==> Unlinking Binary '/usr/local/bin/airspy_info'.
==> Backing App 'Gqrx.app' up to '/usr/local/Caskroom/gqrx/2.11.5/Gqrx.app'.
==> Removing App '/Applications/Gqrx.app'.
==> Purging files for version 2.11.5 of Cask gqrx
Error: Failure while executing; `/usr/bin/xattr -w com.apple.metadata:kMDItemAlternateNames \(\"gqrx\"\) /usr/local/Caskroom/gqrx/2.11.5/gqrx.wrapper.sh` exited with 127. Here's the output:
No such file or directory - /usr/bin/xattr
Follow the instructions here:
  https://github.com/Homebrew/homebrew-cask#reporting-bugs
/usr/local/Homebrew/Library/Homebrew/system_command.rb:176:in `assert_success!'
/usr/local/Homebrew/Library/Homebrew/system_command.rb:53:in `run!'
/usr/local/Homebrew/Library/Homebrew/system_command.rb:29:in `run'
/usr/local/Homebrew/Library/Homebrew/system_command.rb:33:in `run!'
/usr/local/Homebrew/Library/Homebrew/cask/artifact/relocated.rb:75:in `add_altname_metadata'
/usr/local/Homebrew/Library/Homebrew/cask/artifact/symlinked.rb:67:in `create_filesystem_link'
/usr/local/Homebrew/Library/Homebrew/cask/artifact/symlinked.rb:54:in `link'
/usr/local/Homebrew/Library/Homebrew/cask/artifact/binary.rb:9:in `link'
/usr/local/Homebrew/Library/Homebrew/cask/artifact/symlinked.rb:17:in `install_phase'
/usr/local/Homebrew/Library/Homebrew/cask/installer.rb:218:in `block in install_artifacts'
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/set.rb:777:in `each'
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/set.rb:777:in `each'
/usr/local/Homebrew/Library/Homebrew/cask/installer.rb:209:in `install_artifacts'
/usr/local/Homebrew/Library/Homebrew/cask/installer.rb:101:in `install'
/usr/local/Homebrew/Library/Homebrew/cask/cmd/install.rb:22:in `block in run'
/usr/local/Homebrew/Library/Homebrew/cask/cmd/install.rb:16:in `each'
/usr/local/Homebrew/Library/Homebrew/cask/cmd/install.rb:16:in `run'
/usr/local/Homebrew/Library/Homebrew/cask/cmd/abstract_command.rb:36:in `run'
/usr/local/Homebrew/Library/Homebrew/cask/cmd.rb:92:in `run_command'
/usr/local/Homebrew/Library/Homebrew/cask/cmd.rb:158:in `run'
/usr/local/Homebrew/Library/Homebrew/cask/cmd.rb:123:in `run'
/usr/local/Homebrew/Library/Homebrew/cmd/cask.rb:9:in `cask'
/usr/local/Homebrew/Library/Homebrew/brew.rb:103:in `<main>'