conda_install should automatically escape args when they contain version constraints

[smarie]

In Session.conda_install one reads this example:

session.conda_install('--channel=conda-forge', 'dask==2.1.0')

Now consider that we tweak it very lightly:

session.conda_install('--channel=conda-forge', 'dask>2.0.0')

Running it, at least on windows, will

• not take into account the constraint ">2.0.0"
• write a nice but completely useless file named "2.0.0" in the working dir, containing the installation log.

Even worse: reverting the sign ('dask<2.0.0') yields an error:

Command conda install --yes --prefix C:\......\.nox\tests-2-7 --channel=conda-forge dask<2.0.0 failed with exit code 1:
The system cannot find the file specified.
Traceback (most recent call last):
  ...
  File "C:\_dev\python_ws\_Libs_OpenSource\nox\nox\sessions.py", line 293, in conda_install
    **kwargs
  File "C:\_dev\python_ws\_Libs_OpenSource\nox\nox\sessions.py", line 246, in _run
    return nox.command.run(args, env=env, paths=self.bin_paths, **kwargs)
  File "C:\_dev\python_ws\_Libs_OpenSource\nox\nox\command.py", line 125, in run
    raise CommandFailed("Returned code {}".format(return_code))
nox.command.CommandFailed: Returned code 1

This is of course because the > and < operators are handled by the shell and should be escaped. I'll try to propose some fix.