[WIP] "requires" config improvements

[obestwalter]

If requirements are not satisfied the error is now represented by a custom exception and appropriately caught to make the error look like what it is: a misconfiguration of the environment rather than an error in tox.

so this:

tox
ERROR: MissingRequirement: Packages tox-venv, tox-travis need to be installed alongside tox in [...]/bin/python

rather than:

tox                                 (git)-[generic-project/master:613bf7ce2c38+! ]
requirement missing tox-venv
requirement missing tox-travis
Traceback (most recent call last):
  File "[...]/bin/tox", line 11, in <module>
    sys.exit(cmdline())
  File "[...]/lib/python3.7/site-packages/tox/session.py", line 41, in cmdline
    main(args)
  File "[...]/lib/python3.7/site-packages/tox/session.py", line 46, in main
    config = prepare(args)
  File "[...]/lib/python3.7/site-packages/tox/session.py", line 28, in prepare
    config = parseconfig(args)
  File "[...]/lib/python3.7/site-packages/tox/config.py", line 232, in parseconfig
    ParseIni(config, config_file, content)
  File "[...]/lib/python3.7/site-packages/tox/config.py", line 982, in __init__
    self.ensure_requires_satisfied(reader.getlist("requires"))
  File "[...]/lib/python3.7/site-packages/tox/config.py", line 1099, in ensure_requires_satisfied
    raise RuntimeError("not all requirements satisfied, install them alongside tox")
RuntimeError: not all requirements satisfied, install them alongside tox

The docs give explicit info now also, that require is not doing what you might expect if you draw an analogy to the require options in setuptools.setup().

Marked this wip as I am not sure whether we really need a new exception or how to deal in general with our internal exceptions. I feel we should catch them all on the upper level and output a friendly error rather than a traceback (but maybe this should be tackled in another PR).

[codecov]

Codecov Report

Merging #1022 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1022      +/-   ##
==========================================
- Coverage   70.39%   70.36%   -0.03%     
==========================================
  Files          14       14              
  Lines        3486     3486              
  Branches      465      465              
==========================================
- Hits         2454     2453       -1     
  Misses        971      971              
- Partials       61       62       +1

Impacted Files	Coverage Δ
src/tox/interpreters.py	82.73% <ø> (ø)	⬆️
src/tox/config.py	65.1% <100%> (-0.11%)	⬇️
src/tox/session.py	78.12% <100%> (ø)	⬆️
src/tox/exception.py	79.16% <100%> (+0.44%)	⬆️

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[gaborbernat]

Fair catch, however will be rendered not needed by #998. We can merge it in the meantime.

[obestwalter]

ah - didn't see that one yet. Definitely a good way to go. I just started catching up properly today and did not really go through all the issues :)

[gaborbernat]

Looks like an improvement on its own so I'll merge it.