Revert using Python resolves feature (#93)

We decided to not widely publicize resolves in Pants 2.10. Pants's mechanism works great, but using Poetry for lockfile generation is causing several issues:

* `[python-repos]` does not work
* VCS/local requirements don't work
* some transitive deps end up with environment markers that are nonsensical and cause the resolve to fail. There is a workaround, but it's not obvious. This didn't hit example-python, but did hit a user of Pants.

We'll switch to resolves once we can use Pex for lockfile generation: pantsbuild/pants#13964

Author: Eric-Arellano

build-support/generate_constraints.sh

@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+# Copyright 2020 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+# See https://www.pantsbuild.org/v2.0/docs/python-third-party-dependencies.
+
+set -euo pipefail
+
+PYTHON_BIN=python3
+VIRTUALENV=build-support/.venv
+PIP="${VIRTUALENV}/bin/pip"
+CONSTRAINTS_FILE=constraints.txt
+
+"${PYTHON_BIN}" -m venv "${VIRTUALENV}"
+"${PIP}" install pip --upgrade
+"${PIP}" install -r <(./pants dependencies --type=3rdparty ::)
+echo "# Generated by build-support/generate_constraints.sh on $(date)" > "${CONSTRAINTS_FILE}"
+"${PIP}" freeze --all >> "${CONSTRAINTS_FILE}"

constraints.txt

@@ -0,0 +1,5 @@
+# Generated by build-support/generate_constraints.sh on Fri Nov 12 13:02:01 MST 2021
+ansicolors==1.1.8
+pip==21.3.1
+setuptools==56.2.0
+types-setuptools==57.4.2

lockfile.txt

@@ -26,17 +26,18 @@ repo_id = "3B1D361B-E9F1-49A8-B761-03DCC41FD58E"
 root_patterns = ["/"]
 
 [python]
-# The default interpreter compatibility for code in this repo. Individual targets can override
+# The default interpreter constraints for code in this repo. Individual targets can override
 #  this with the `interpreter_constraints` field. See
 #  https://www.pantsbuild.org/docs/python-interpreter-compatibility.
 #
+# It's usually a good idea to pin this to a particular version, e.g. `==3.9.*`. We only use a range
+# so that this example works on many different machines.
+#
 # If you use Apple Silicon, set to a value like `>=3.9`.
 interpreter_constraints = [">=3.7"]
 
-# Enable the "resolves" mechanism, which allows you to have multiple lockfiles and for Pants to
-# generate the lockfiles for you. See https://www.pantsbuild.org/docs/python-third-party-dependencies.
-enable_resolves = true
-resolves = { python-default = "lockfile.txt"}
+# Use a constraints file. See https://www.pantsbuild.org/docs/python-third-party-dependencies.
+requirement_constraints = "constraints.txt"
 
 [python-bootstrap]
 # We search for interpreters on both on the $PATH and in the `$(pyenv root)/versions` folder.