Improvements for ST4

What's Changed

• ST4 fixes by @keith-hall in #46
  • completions now look prettier
  • using Python 3.8 plugin host
  • lxml was upgraded (now using Package Control 4 libraries), which has a breaking change that all namespaces in the xpath query have to contain a non-blank prefix

Full Changelog: v2.6.1...ST4-v3.0.0