Go back to using resolves, but hedge about lockfile generation (#94)

Reverts #93.

As discussed in https://pantsbuild.slack.com/archives/C0D7TNJHL/p1646948835870139, we believe that multiple resolves even with manual generation is still an improvement over `[python].requirement_constraints`. The hope is that most users can use `generate-lockfiles` directly, but if not, our new docs continue to give some techniques for manual lockfile generation. 

Our 2.10 docs no longer discuss `requirement_constraints`, in favor of resolves.

Author: Eric-Arellano

build-support/generate_constraints.sh

@@ -0,0 +1,27 @@
+# This lockfile was autogenerated by Pants. To regenerate, run:
+#
+#    ./pants generate-lockfiles --resolve=python-default
+#
+# --- BEGIN PANTS LOCKFILE METADATA: DO NOT EDIT OR REMOVE ---
+# {
+#   "version": 2,
+#   "valid_for_interpreter_constraints": [
+#     "CPython>=3.7"
+#   ],
+#   "generated_with_requirements": [
+#     "ansicolors==1.1.8",
+#     "setuptools<57,>=56.2.0",
+#     "types-setuptools<58,>=56.2.0"
+#   ]
+# }
+# --- END PANTS LOCKFILE METADATA ---
+
+ansicolors==1.1.8 \
+    --hash=sha256:00d2dde5a675579325902536738dd27e4fac1fd68f773fe36c21044eb559e187 \
+    --hash=sha256:99f94f5e3348a0bcd43c82e5fc4414013ccc19d70bd939ad71e0133ce9c372e0
+setuptools==56.2.0; python_version >= "3.6" \
+    --hash=sha256:bc30153eec47d82f20c6f5e1a13d4ee725c6deb7013a67557f89bfe5d25235c4 \
+    --hash=sha256:7bb5652625e94e73b9358b7ed8c6431b732e80cf31f4e0972294c64f0e5b849e
+types-setuptools==57.4.9 \
+    --hash=sha256:536ef74744f8e1e4be4fc719887f886e74e4cf3c792b4a06984320be4df450b5 \
+    --hash=sha256:948dc6863373750e2cd0b223a84f1fb608414cde5e55cf38ea657b93aeb411d2

constraints.txt

@@ -2,7 +2,7 @@
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
 [GLOBAL]
-pants_version = "2.10.0rc0"
+pants_version = "2.10.0rc3"
 backend_packages.add = [
   "pants.backend.python",
   "pants.backend.python.lint.docformatter",
@@ -36,11 +36,15 @@ root_patterns = ["/"]
 # If you use Apple Silicon, set to a value like `>=3.9`.
 interpreter_constraints = [">=3.7"]
 
-# Use a constraints file. See https://www.pantsbuild.org/docs/python-third-party-dependencies.
-requirement_constraints = "constraints.txt"
+# Enable the "resolves" mechanism, which turns on lockfiles for user code. See
+# https://www.pantsbuild.org/docs/python-third-party-dependencies. This also adds the
+# `generate-lockfiles` goal for Pants to generate the lockfile for you, although in some
+# situations you may need to manually generate it, as explained in the docs.
+enable_resolves = true
+resolves = { python-default = "lockfile.txt"}
 
 [python-bootstrap]
-# We search for interpreters on both on the $PATH and in the `$(pyenv root)/versions` folder.
+# We search for interpreters both on the $PATH and in the `$(pyenv root)/versions` folder.
 #  If you're using macOS, you may want to leave off the <PATH> entry to avoid using the
 #  problematic system Pythons. See
 #  https://www.pantsbuild.org/docs/python-interpreter-compatibility#changing-the-interpreter-search-path.