## Description
Fixed gevent error in tox tests:
```
Collecting gevent<20.10,>=20.9
Using cached gevent-20.9.0.tar.gz (5.8 MB)
Installing build dependencies: started
Installing build dependencies: finished with status 'done'
Getting requirements to build wheel: started
Getting requirements to build wheel: finished with status 'error'
error: subprocess-exited-with-error
× Getting requirements to build wheel did not run successfully.
│ exit code: 1
╰─> [60 lines of output]
warning: src/gevent/resolver/cares.pyx:38:0: The 'DEF' statement is deprecated and will be removed in a future Cython version. Consider using global variables, constants, and in-place literals instead. See cython/cython#4310
warning: src/gevent/resolver/cares.pyx:40:0: The 'DEF' statement is deprecated and will be removed in a future Cython version. Consider using global variables, constants, and in-place literals instead. See cython/cython#4310
warning: src/gevent/resolver/cares.pyx:41:0: The 'DEF' statement is deprecated and will be removed in a future Cython version. Consider using global variables, constants, and in-place literals instead. See cython/cython#4310
Compiling src/gevent/resolver/cares.pyx because it changed.
[1/1] Cythonizing src/gevent/resolver/cares.pyx
warning: src/gevent/libev/corecext.pyx:325:0: The 'DEF' statement is deprecated and will be removed in a future Cython version. Consider using global variables, constants, and in-place literals instead. See cython/cython#4310
warning: src/gevent/libev/corecext.pyx:783:0: The 'DEF' statement is deprecated and will be removed in a future Cython version. Consider using global variables, constants, and in-place literals instead. See cython/cython#4310
warning: src/gevent/libev/corecext.pyx:785:0: The 'DEF' statement is deprecated and will be removed in a future Cython version. Consider using global variables, constants, and in-place literals instead. See cython/cython#4310
warning: src/gevent/libev/corecext.pyx:787:0: The 'DEF' statement is deprecated and will be removed in a future Cython version. Consider using global variables, constants, and in-place literals instead. See cython/cython#4310
warning: src/gevent/libev/corecext.pyx:791:0: The 'DEF' statement is deprecated and will be removed in a future Cython version. Consider using global variables, constants, and in-place literals instead. See cython/cython#4310
Compiling src/gevent/libev/corecext.pyx because it changed.
[1/1] Cythonizing src/gevent/libev/corecext.pyx
Compiling src/gevent/_greenlet_primitives.py because it changed.
[1/1] Cythonizing src/gevent/_greenlet_primitives.py
Compiling src/gevent/_hub_primitives.py because it changed.
[1/1] Cythonizing src/gevent/_hub_primitives.py
Compiling src/gevent/_hub_local.py because it changed.
[1/1] Cythonizing src/gevent/_hub_local.py
Compiling src/gevent/_waiter.py because it changed.
[1/1] Cythonizing src/gevent/_waiter.py
warning: src/gevent/_gevent_cgreenlet.pxd:112:33: Declarations should not be declared inline.
Error compiling Cython file:
------------------------------------------------------------
...
cdef load_traceback
cdef Waiter
cdef wait
cdef iwait
cdef reraise
cpdef GEVENT_CONFIG
^
------------------------------------------------------------
src/gevent/_gevent_cgreenlet.pxd:181:6: Variables cannot be declared with 'cpdef'. Use 'cdef' instead.
Compiling src/gevent/greenlet.py because it changed.
[1/1] Cythonizing src/gevent/greenlet.py
Traceback (most recent call last):
```
## Reviewer Checklist
- [ ] Title is accurate.
- [ ] Description motivates each change.
- [ ] No unnecessary changes were introduced in this PR.
- [ ] PR cannot be broken up into smaller PRs.
- [ ] Avoid breaking [API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces) changes unless absolutely necessary.
- [ ] Tests provided or description of manual testing performed is included in the code or PR.
- [ ] Release note has been added for fixes and features, or else `changelog/no-changelog` label added.
- [ ] All relevant GitHub issues are correctly linked.
- [ ] Backports are identified and tagged with Mergifyio.
- [ ] Add to milestone.
(cherry picked from commit 5a6ac70)
Co-authored-by: Alberto Vara <alberto.vara@datadoghq.com>
(cherry picked from commit 523793d)