[RFE] Add support for applying constraints

[sagarun]

pip supports applying upper/lower constraints on a requirements.txt file however invirtualenv doesn't. There should be a way to do this

Here is how upper constraints are applied in a tox.ini

[testenv]
install_command = pip install -U {opts} {packages}
deps = 
    -c{env:UPPER_CONSTRAINTS_FILE:upper-constraints.txt}
    -r{toxinidir}/requirements.txt
    -r{toxinidir}/test-requirements.txt

[dwighthubbard]

Are you thinking the configuration could specify constraints in the same way as dependencies are defined?