Insert proper patch for py37+ support on dataclasses

The patch is proposed on https://github.com/ericvsmith/dataclasses/pull/161 on goes beyond what I did for this package in the past.

It (gracefully) falls-back to the built-in dataclasses on python 3.7+ instead of replacing that library.

This package was added here to stop pytorch=1.7 from complaining during installation. pytorch depends (wrongly) on dataclasses for any version of Python, whereas this should be done, only for python<3.7. A fix will only be available on pytorch=1.8 (but that may take another couple of weeks to show).

Closes bob/bob.ip.binseg#5

conda/dataclasses/allow-py37-ge-environments.patch

+From 178831f257a0d7d1b112b77067890ba5fd44f9a4 Mon Sep 17 00:00:00 2001
+From: Jon Dufresne <jon.dufresne@gmail.com>
+Date: Thu, 3 Sep 2020 17:43:45 -0700
+Subject: [PATCH] Allow installing  on Python 3.7+ environments
+
+On Python 3.7+ environments, use the builtin dataclasses module instead
+of shadowing it. This is handled in the dataclasses/__init__.py which
+sets the module in sys.modules.
+
+This allows projects to pin dependencies that are then run in both
+Python 3.6 and 3.7+ environments using tools such as pip-tools.
+
+Previously, if dependencies were pinned for Python 3.6, the dataclasses
+would be used, but fail to install on Python 3.7+ environments. On the
+other hand, if dependencies were pinned for Python 3.7+, dataclasses
+would not be available in Python 3.6 environments.
+
+Projects typically pin dependencies using the oldest supported Python.
+
+dataclasses has recently become a dependency of the popular tool Black,
+which is how this issue was discovered.
+
+Fixes #153
+---
+ dataclasses/__init__.py                      | 17 +++++++++++++++++
+ dataclasses.py => dataclasses/dataclasses.py |  0
+ setup.py                                     |  9 +++++++--
+ test/test_dataclasses.py                     | 11 +++++++++++
+ 4 files changed, 35 insertions(+), 2 deletions(-)
+ create mode 100644 dataclasses/__init__.py
+ rename dataclasses.py => dataclasses/dataclasses.py (100%)
+
+diff --git a/dataclasses/__init__.py b/dataclasses/__init__.py
+new file mode 100644
+index 0000000..7818f17
+--- /dev/null
++++ b/dataclasses/__init__.py
+@@ -0,0 +1,17 @@
++import importlib.util
++import os
++import sys
++
++for path in sys.path:
++    file_path = os.path.join(path, 'dataclasses.py')
++    if os.path.exists(file_path):
++        # Load the builtin module instead.
++        break
++else:
++    file_path = os.path.join(os.path.dirname(__file__), 'dataclasses.py')
++
++module_name = 'dataclasses'
++spec = importlib.util.spec_from_file_location(module_name, file_path)
++module = importlib.util.module_from_spec(spec)
++sys.modules[module_name] = module
++spec.loader.exec_module(module)
+diff --git a/dataclasses.py b/dataclasses/dataclasses.py
+similarity index 100%
+rename from dataclasses.py
+rename to dataclasses/dataclasses.py
+diff --git a/setup.py b/setup.py
+index e623a2a..84f1f68 100644
+--- a/setup.py
++++ b/setup.py
+@@ -17,8 +17,13 @@
+         "Intended Audience :: Developers",
+         "Topic :: Software Development :: Libraries :: Python Modules",
+         "License :: OSI Approved :: Apache Software License",
++        "Programming Language :: Python",
++        "Programming Language :: Python :: 3",
++        "Programming Language :: Python :: 3 :: Only",
+         "Programming Language :: Python :: 3.6",
++        "Programming Language :: Python :: 3.7",
++        "Programming Language :: Python :: 3.8",
+     ],
+-    py_modules=["dataclasses"],
+-    python_requires=">=3.6, <3.7",
++    packages=["dataclasses"],
++    python_requires=">=3.6",
+ )
+diff --git a/test/test_dataclasses.py b/test/test_dataclasses.py
+index 741d0d6..5667f34 100644
+--- a/test/test_dataclasses.py
++++ b/test/test_dataclasses.py
+@@ -6,6 +6,8 @@
+ 
+ import pickle
+ import inspect
++import os
++import sys
+ import unittest
+ from unittest.mock import Mock
+ from typing import ClassVar, Any, List, Union, Tuple, Dict, Generic, TypeVar, Optional
+@@ -3056,6 +3058,15 @@ def __post_init__(self, y):
+ 
+     ##     replace(c, x=5)
+ 
++    def test_import_shadows_builtin(self):
++        mod_path = dataclasses.__file__
++        self.assertTrue(mod_path.endswith('dataclasses.py'))
++        py36_suffix = os.path.join('dataclasses', 'dataclasses.py')
++        if sys.version_info < (3, 7):
++            self.assertTrue(mod_path.endswith(py36_suffix))
++        else:
++            self.assertFalse(mod_path.endswith(py36_suffix))
++
+ 
+ if __name__ == '__main__':
+     unittest.main()