docs: mention editable mode installation

Make resolve max rounds configurable

change dependencies to a list of PEP 508 strings

Test pip 21.0

Fix #231

Change the compatible strategy to use ~= operator

Feature #226 Introducing ${PROJECT_ROOT} environment variable

Close #227

Support specifying Python interpreter by `pdm use <path-to-python-root>`.

Auto disable PEP 582 when a venv-like python is given

Feature/auth