Remove use_2to3 backward compat for Setuptools

Odds are this was never used by any PBR-based projects, but was
included from the beginning to support a feature of Setuptools for a
utility which has been long since discouraged. As of Setuptools
46.2.0 and Python 3.9 it's generating deprecation warnings, and is
slated for removal from the Python 3.10 standard library.

https://bugs.python.org/issue40360
pypa/setuptools#2086

Change-Id: I6972984f29413d2534d802b142fa9323a7e4a4a3

pbr/util.py

@@ -128,7 +128,6 @@
     "cmdclass": ("global", "commands"),
     # Not supported in distutils2, but provided for
     # backwards compatibility with setuptools
-    "use_2to3": ("backwards_compat", "use_2to3"),
     "zip_safe": ("backwards_compat", "zip_safe"),
     "tests_require": ("backwards_compat", "tests_require"),
     "dependency_links": ("backwards_compat",),
@@ -158,7 +157,7 @@
 MAP_FIELDS = ("project_urls",)
 
 # setup() arguments that contain boolean values
-BOOL_FIELDS = ("use_2to3", "zip_safe", "include_package_data")
+BOOL_FIELDS = ("zip_safe", "include_package_data")
 
 CSV_FIELDS = ()
 

releasenotes/notes/use_2to3-removal-ac48bf9fbfa049b1.yaml

@@ -0,0 +1,9 @@
+---
+other:
+  - |
+    The 2to3 conversion utility has been long discouraged in favor of writing
+    multi-version-capable scripts. As of Setuptools 46.2.0 it's deprecated and
+    slated for removal from the Python 3.10 standard library. Projects which
+    still need it are encouraged to perform conversion prior to packaging. See
+    https://bugs.python.org/issue40360 and
+    https://github.com/pypa/setuptools/issues/2086 for more details.