fix(deps): fix issue with attrs and contextlib2 version constraints for Python 2.7 (backport #4357) (#4382)

* fix(deps): fix issue with attrs and contextlib2 version constraints for Python 2.7 (#4357)

## Description

The package attrs 22.0 dropped support for Python 2.7. Also pip gets confused on the conxtextlib2 (backport library) version requirements.

## Motivation

While trying to integrate DD with our product (which is legacy python 2.7) these build problems were encountered.

## Testing strategy
In python 2.7 virtualenv (which is separated from the internet), run the pip install ddtrace.

## Reviewer Checklist
- [x] Title is accurate.
- [x] Description motivates each change.
- [x] No unnecessary changes were introduced in this PR.
- [x] Avoid breaking [API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces) changes unless absolutely necessary.
- [x] Tests provided or description of manual testing performed is included in the code or PR.
- [x] Release note has been added for fixes and features, or else `changelog/no-changelog` label added.
- [x] All relevant GitHub issues are correctly linked.
- [x] Backports are identified and tagged with Mergifyio.

(cherry picked from commit 8bdafe5)

# Conflicts:
#	setup.py

* Update setup.py

Co-authored-by: Peter Cooner <[email protected]>
Co-authored-by: Brett Langdon <[email protected]>

Author: 3 people

releasenotes/notes/Fix-version-constraints-for-Py27-e3a0069f43a84c16.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    Fix issue with ``attrs`` and ``contextlib2`` version constraints for Python 2.7.

setup.py

@@ -250,7 +250,9 @@ def get_exts_for(name):
         "protobuf>=3,<4.0; python_version=='3.6'",
         "protobuf>=3,<3.18; python_version<'3.6'",
         "tenacity>=5",
-        "attrs>=19.2.0",
+        "attrs>=19.2.0; python_version>'2.7'",
+        "attrs>=19.2.0,<22; python_version=='2.7'",
+        "contextlib2<1.0; python_version=='2.7'",
         "cattrs",
         "six>=1.12.0",
         "typing_extensions",