Critical setuptools issue resolved x 3.

This commit finalizes the resolution applied by the prior commit to
both the "develop" and "install" setuptools subcommands. Previously,
this resolution applied to only the latter.

Specifically, this commit:

* Mandatory dependencies bumped:
  * setuptools >= 36.7.2. A recent version of setuptools is now
    required, as this application requires the
    setuptools.command.easy_install.ScriptWriter.get_args() class
    method and thus the oldest version of setuptools with this method.
    Since official setuptools documentation fails to specify the exact
    version that first defined this method, we fallback to a
    sufficiently old version from 2017 known to do so.
* Generalized the "setup.py" submodule:
  * Called the newly defined
    betse_setup.buputil.die_unless_setuptools_version_at_least()
    validator to ensure the currently installed version of setuptools
    satisfies all installation-time requirements.
* Generalized the "betse.metadeps" submodule:
  * Defined a new "SETUPTOOLS_VERSION_MIN" global, providing the minimum
    version of "setuptools"required at both install- and runtime.
    Previously, this global was inaccessibly hardcoded into the
    coarse-grained "RUNTIME_MANDATORY" global.
* Generalized the "betse_setup.buputil" submodule.
  * Defined a new die_unless_setuptools_version_at_least() validator,
    copied verbatim from the "betsee.beuputil" submodule.
  * Minimized this submodule to the smallest set of requisite functions
    required by the top-level "setup.py" script.

betse/lib/setuptools/command/supcommand.py

@@ -28,7 +28,7 @@ def add_subcommand(
     #FIXME: Type-check each such command to be a "Command". For unknown
     #reasons, Python is currently complaining that "freeze_dir" is not a
     #"Command", despite clearly being a "Command". </collective_shrug>
-    *subcommands
+    *subcommands,
 ) -> None:
     '''
     Define one custom :mod:`setuptools` subcommand for each passed class,

betse/metadeps.py

@@ -26,6 +26,23 @@
 
 from collections import namedtuple
 
+# ....................{ LIBS ~ install : mandatory        }....................
+# This public global is externally referenced by "setup.py".
+SETUPTOOLS_VERSION_MIN = '36.7.2'
+'''
+Minimum version of :mod:`setuptools` required at both application install- and
+runtime as a human-readable ``.``-delimited string.
+
+Motivation
+----------
+This application requires the
+:meth:`setuptools.command.easy_install.ScriptWriter.get_args` class method and
+hence at least the oldest version of :mod:`setuptools` to have this method.
+Since official setuptools documentation fails to specify the exact version that
+first defined this method, we fallback to a sufficiently old version from 2017
+known to define this method.
+'''
+
 # ....................{ LIBS ~ runtime : mandatory        }....................
 #!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 # WARNING: Changes to this subsection *MUST* be synchronized with:
@@ -38,19 +55,9 @@
 #!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 
 RUNTIME_MANDATORY = {
-    #FIXME: Extract into a global constant and validate at installation time.
-    #See the BETSEE codebase for requisite logic.
-
     # setuptools is currently required at both install and runtime. At runtime,
-    # setuptools is used to validate that dependencies are available. Notably:
-    #
-    # * setuptools 36.7.2 is the oldest version for which the
-    #   setuptools.command.easy_install.ScriptWriter.get_script_args() class
-    #   method is known to have been deprecated. Technically, older versions
-    #   probably exist. Since official setuptools documentation fails to
-    #   specify the exact version that first deprecated this method, we have no
-    #   sane recourse but to accept this "old enough" version from 2017.
-    'setuptools': '>= 36.7.2',
+    # setuptools is used to validate that dependencies are available.
+    'setuptools': '>= ' + SETUPTOOLS_VERSION_MIN,
 
     # Dependencies directly required by this application. Notably:
     #