Support backticks within deps

[jayvdb]

It should be straight forward to add backticks within deps, and doesnt present any conflict with existing syntax as backticks are invalid requirements.txt syntax.

[jayvdb]

So this works in at least tox 3.8+

[testenv:deps]
setenv =
  DEPS=distro
deps =
  {env:DEPS}
commands =
  distro --help

However if setenv of DEPS uses backticks, installdeps fails because it occurs before the backticks handling which is done later in the hook tox_runtest_pre.

So a bit of re-structure is needed to use hook tox_testenv_install_deps to perform the backtick handling sooner. Using the earlier hook tox_testenv_create might also be possible. Removing the backticks asap is very likely to be better for interoperability with other tox plugins.