chore: Use constraints.txt for lower bound testing

[matthewfeickert]

Description

As suggested by @henryiii on Twitter, rename the lower-bound-requirements.txt to constraints.txt and use it as a pip constraints file instead of as a requirements.txt.

$ pip install --help | grep constraints
  -c, --constraint <file>     Constrain versions using the given constraints

and from the pip docs on constraints files:

Constraints files are requirements files that only control which version of a requirement is installed, not whether it is installed or not. Their syntax and contents is a subset of Requirements Files, with several kinds of syntax not allowed: constraints must have a name, they cannot be editable, and they cannot specify extras. In terms of semantics, there is one key difference: Including a package in a constraints file does not trigger installation of the package.

As also suggested by @henryiii in Issue #1685, move the constraints.txt under the tests directory to reduce clutter in the top level directory.

As constraints files work like requirements files they are still able to follow --find-links URLs and so can download old jaxlib releases from https://storage.googleapis.com/jax-releases/jax_releases.html. 👍

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Use pip's options for constraints files to test lower bounds of install
requirements.
   - Rename 'lower-bound-requirements.txt' to 'constraints.txt'.
   - Use pip's --constraint option while installing the 'test' extra that
     includes all backends and user extras.
   - c.f. https://pip.pypa.io/en/stable/user_guide/#constraints-files
   - Note that transitioning from a requirements file to a constraints file
     works only because the 'test' extra already specifies all of the
     libraries found in the constraints file. Constraints files do NOT
     ensure that a library is installed. They only ensure that if it IS
     installed that it meets the constraints specified in the constraints
     files.
     > Constraints files are requirements files that only control which
     > version of a requirement is installed, not whether it is installed
     > or not.
* Move the constraints.txt under the tests/ directory to reduce clutter
in the top level of the repository.

[codecov]

Codecov Report

Merging #1713 (4da7cba) into master (6103f13) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1713   +/-   ##
=======================================
  Coverage   98.12%   98.12%           
=======================================
  Files          64       64           
  Lines        4269     4269           
  Branches      682      682           
=======================================
  Hits         4189     4189           
  Misses         46       46           
  Partials       34       34           

Flag	Coverage Δ
contrib	26.25% <ø> (ø)
doctest	60.59% <ø> (ø)
unittests	96.18% <ø> (ø)

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[matthewfeickert]

If I diff the output of pip list for the lower-bound-requirements.txt vs. constraints.txt I get

$ diff --color -u /tmp/lower-bound-requirements-output.txt /tmp/constraints-output.txt 
--- /tmp/lower-bound-requirements-output.txt	2021-11-29 20:44:15.692252460 -0600
+++ /tmp/constraints-output.txt	2021-11-29 20:44:41.536190380 -0600
@@ -1,11 +1,12 @@
 absl-py                 1.0.0
 ansiwrap                0.8.4
+appdirs                 1.4.4
 argcomplete             1.12.3
 argon2-cffi             21.1.0
 astunparse              1.6.3
 attrs                   21.2.0
 backcall                0.2.0
-black                   21.11b1
+black                   20.8b0
 bleach                  4.1.0
 cachetools              4.2.4
 certifi                 2021.10.8
@@ -80,7 +81,6 @@
 pickleshare             0.7.5
 Pillow                  8.4.0
 pip                     21.3.1
-platformdirs            2.4.0
 pluggy                  1.0.0
 prometheus-client       0.12.0
 prompt-toolkit          3.0.23

which seems acceptable as none of those are specified in either files. So I'll merge this now that there is 1 approval.