Bump flake8 from 3.6.0 to 3.8.1

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.6.0 to 3.8.1.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.6.0...3.8.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements-tests.txt

@@ -3,5 +3,5 @@ attrs==19.3.0
 pytest==5.4.2
 pytest-cov==2.8.1
 mock==2.0.0
-flake8==3.6.0
+flake8==3.8.1
 pyflakes==2.2.0

sshuttle/ssnet.py

@@ -61,14 +61,14 @@ NET_ERRS = [errno.ECONNREFUSED, errno.ETIMEDOUT,
             errno.ECONNRESET]
 
 
-def _add(l, elem):
-    if elem not in l:
-        l.append(elem)
+def _add(socks, elem):
+    if elem not in socks:
+        socks.append(elem)
 
 
-def _fds(l):
+def _fds(socks):
     out = []
-    for i in l:
+    for i in socks:
         try:
             out.append(i.fileno())
         except AttributeError: