Adds nixpkgs_python_configure

[aherrmann]

• Define nixpkgs constraint
  This is intended to move the nixpkgs platform constraint defined in rules_haskell into rules_nixpkgs so that it can be shared. It is used by nixpkgs_unix_configure in the exec_compatible_with attribute of the generated toolchain.
• Update Bazel to version 1.1.0
  The update to Bazel 1.1.0 on rules_haskell (anything >0.29 really) motivates the introduction of nixpkgs_python_configure. See Update to Bazel 1.1 rules_haskell#1142.
• Add nixpkgs_python_configure

  Define and register a Python toolchain provided by nixpkgs.
  Creates nixpkgs_packages for Python 2 or 3 py_runtime instances and a
  corresponding py_runtime_pair and toolchain. The toolchain is
  automatically registered and uses the constraint:
  "@io_tweag_rules_nixpkgs//nixpkgs/constraints:nixpkgs"

• Adds a test-case for nixpkgs_python_configure.

This PR takes the definition of the nixpkgs constraint and the Bazel update out of #95. It should take precedence, as nixpkgs_python_configure is required to update Bazel in rules_haskell, which in turn is required to use nixpkgs_unix_configure in rules_haskell.

[guibou]

LGTM. Congratulation for understanding the toolchains filtering mechanisms.