Fix Travis CI build error on OSX - fails with pip3: command not found

The Travis CI builds are failing with the following messages:

    sudo -H pip3 install --upgrade pip setuptools wheel
    sudo: pip3: command not found

    The command "if [ "$TRAVIS_OS_NAME" = "osx" ]; then bash package/prepare_osx.sh; fi" failed and exited with 1 during .

It appears that homebrew recipes for Python3 and pip3 install and
invocation keep changing (see Homebrew version after 1.5).

https://stackoverflow.com/questions/17271319/how-do-i-install-pip-on-macos-or-os-x

package/prepare_osx.sh

@@ -4,6 +4,7 @@ brew update
 # upgrade to python 3.x
 brew upgrade python
 brew install enchant
+brew postinstall python # this installs pip
 sudo -H pip3 install --upgrade pip setuptools wheel
 pip3 install pyinstaller PyQt5 lxml pyenchant
 brew install qt hunspell