.jshintrc

tests: use venv on Python 3

This test was failing in some Python 3 environments because
`$PYTHON -m virtualenv` was somehow resulting in Python 2
being used. Why, I'm not sure.

Python 3 includes virtualenv in the standard library as the
`venv` module.

This commit changes test-install.t to use `$PYTHON -m venv` on
Python 3 and `$PYTHON -m virtualenv` on Python 2 (if available).

I chose to make some test output duplicated because we can't
have nested conditionals and there is no easy way to express
ORing of hghave checks.

Differential Revision: https://phab.mercurial-scm.org/D7224

{
    // Enforcing
    "eqeqeq"        : true,     // true: Require triple equals (===) for comparison
    "forin"         : true,     // true: Require filtering for..in loops with obj.hasOwnProperty()
    "freeze"        : true,     // true: prohibits overwriting prototypes of native objects such as Array, Date etc.
    "nonbsp"        : true,     // true: Prohibit "non-breaking whitespace" characters.
    "undef"         : true,     // true: Require all non-global variables to be declared (prevents global leaks)

    // Environments
    "browser"       : true      // Web Browser (window, document, etc)
}