fix(mypy): avoid parsing redis asyncio files with Python 2 (#3868)

## Description

Fixes #3845

## Checklist
- [ ] Title must conform to [conventional commit](https://github.com/conventional-changelog/commitlint/tree/master/%40commitlint/config-conventional).
- [ ] Add additional sections for `feat` and `fix` pull requests.
- [ ] Ensure tests are passing for affected code.
- [ ] [Library documentation](https://github.com/DataDog/dd-trace-py/tree/1.x/docs) and/or [Datadog's documentation site](https://github.com/DataDog/documentation/) is updated. Link to doc PR in description.

## Testing strategy

We do not as yet run Python 2 type checking so this will have to be tested manually.

## Reviewer Checklist
- [x] Title is accurate.
- [x] Description motivates each change.
- [x] No unnecessary changes were introduced in this PR.
- [x] PR cannot be broken up into smaller PRs.
- [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.
- [ ] Add to milestone.

(cherry picked from commit d2ed33a)

ddtrace/contrib/redis/patch.py

@@ -1,4 +1,5 @@
 import redis
+from six import PY3
 
 from ddtrace import config
 from ddtrace.vendor import wrapt
@@ -36,7 +37,8 @@ def patch():
         _w("redis", "Redis.pipeline", traced_pipeline)
         _w("redis.client", "Pipeline.execute", traced_execute_pipeline)
         _w("redis.client", "Pipeline.immediate_execute_command", traced_execute_command)
-        if redis.VERSION >= (4, 2, 0):
+        # Avoid mypy invalid syntax errors when parsing Python 2 files
+        if PY3 and redis.VERSION >= (4, 2, 0):
             from .asyncio_patch import traced_async_execute_command
             from .asyncio_patch import traced_async_execute_pipeline
 

docs/spelling_wordlist.txt

@@ -107,6 +107,7 @@ mongoengine
 multiline
 multiprocess
 multithreaded
+mypy
 mysql
 mysqlclient
 mysqldb

releasenotes/notes/fix-mypy-py2-redis-4c8d3b4e461c6074.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    mypy: Avoid parsing redis asyncio files when type checking Python 2