Support constraints in piplite

[bollwyvl]

Problem

micropip 0.9.0 was just released, which support constraints (e.g. iff package x is installed it must be version y or from URL z). This will be very helpful for jupyterlite site builders as the (likely filtered) output of pip list --format=freeze would get us a step closer to reproducible envs. These won't somehow magically participate in auto-import, as top-level names would still not be available.

When a version of pyodide is released which contains this (or a user builds a custom pyodide distribution containing it), piplite and the %pip magic won't yet support them, and as a "major" version upgrade containing other significant changes (such as vendoring packaging), might just break.

Proposed Solution

• add constraints to various wrapper signatures around micropip.install(..., constraints=[]) and set_constraints
• add %pip install -c/--constraint
  • for pip, this is always a file, but we already support -r/--requirements, so the existing logic should work
• add a way to set default constraints at build time
  • discussed previously, adding a pipliteInstallOptions could allow for setting all the (non-dependency) options:

    {
      "pipliteInstallOptions": {
        "constraints": [
          "ipywidgets ==8.1.2"
        ],
        "index_urls": [],
        "keep_going": true
      }
    }

Additional context

• Add support for constraints, install wheels before pyodide packages pyodide/micropip#181