Update installation instructions to use python3 instead of pip
Update installation instructions to use python3 instead of pip

Follows official Python recommendations for installing modules with python3 [1]
instead of pip to address issues reported by users who tried to use vanilla pip
install where pip was linked to a Python 2 installation and the installation
failed.

[1] https://docs.python.org/3/installing/index.html#basic-usage

nextstrain/augur,blab/nextstrain-augur,nextstrain/augur,nextstrain/augur