Update CI to use pip v19+

[d3r3kk]

The update to pip v19.0 has caused our current CI pipelines to stop working (see #4082). We need to update our CI scripts to not pin pip<19 for the long term.

The gulp script that runs installPythonLibs falls over when installing PTVSD for example, as it uses two flags that are not usable in this version of pip:

Flag 1: --disable-version-check

Running this in a shell with pip 19.0 installed:

python -m pip --disable-version-check install -t .\pythonFiles\lib\python --no-cache-dir --implementation py --no-deps --upgrade ptvsd==4.2.0

results in this:

Usage:
  C:\dev\github\d3r3kk\vscode-python\.venv\Scripts\python.exe -m pip <command> [options]

no such option: --disable-version-check

Flag 2: --no-cache-dir

Running this in a shell with pip 19.0 installed:

python -m pip install -t .\pythonFiles\lib\python --no-cache-dir --implementation py --no-deps --upgrade ptvsd==4.2.0

results in this:

Exception:
Traceback (most recent call last):
  File "C:\dev\github\d3r3kk\vscode-python\.venv\lib\site-packages\pip\_internal\cli\base_command.py", line 176, in main
    status = self.run(options, args)
  File "C:\dev\github\d3r3kk\vscode-python\.venv\lib\site-packages\pip\_internal\commands\install.py", line 346, in run
    session=session, autobuilding=True
  File "C:\dev\github\d3r3kk\vscode-python\.venv\lib\site-packages\pip\_internal\wheel.py", line 848, in build
    assert building_is_possible
AssertionError

This breaks the installed version of pip here:

        :param unpack: If True, replace the sdist we built from with the
            newly built wheel, in preparation for installation.
        :return: True if all the wheels built correctly.
        """
        # TODO: This check fails if --no-cache-dir is set. And yet we
        #       might be able to build into the ephemeral cache, surely?
        building_is_possible = self._wheel_dir or (
            autobuilding and self.wheel_cache.cache_dir
        )
        assert building_is_possible

[d3r3kk]

Upstream issue pypa/pip#6158

Once that is resolved (or guidance for how to properly resolve it is obtained) then we can stop pinning pip at <19

[d3r3kk]

19.01 is up now, should check if this works now.

[kimadeline]

CI is working with pip 19.2.3: