SystemError: error return without exception set

I have same issue from #4367 
```
Exception ignored in: <generator object urlsplit.<locals>.<genexpr> at 0x1087bdca8>
Traceback (most recent call last):
  File "/usr/local/Cellar/python/3.6.4_4/Frameworks/Python.framework/Versions/3.6/lib/python3.6/urllib/parse.py", line 431, in <genexpr>
    if not rest or any(c not in '0123456789' for c in rest):
SystemError: error return without exception set
Exception ignored in: <generator object urlsplit.<locals>.<genexpr> at 0x108788830>
Traceback (most recent call last):
  File "/usr/local/Cellar/python/3.6.4_4/Frameworks/Python.framework/Versions/3.6/lib/python3.6/urllib/parse.py", line 431, in <genexpr>
    if not rest or any(c not in '0123456789' for c in rest):
SystemError: error return without exception set
Exception ignored in: <generator object urlsplit.<locals>.<genexpr> at 0x108788360>
Traceback (most recent call last):
  File "/usr/local/Cellar/python/3.6.4_4/Frameworks/Python.framework/Versions/3.6/lib/python3.6/urllib/parse.py", line 431, in <genexpr>
    if not rest or any(c not in '0123456789' for c in rest):
SystemError: error return without exception set
Exception ignored in: <generator object iter_slices at 0x108788360>
Traceback (most recent call last):
  File "~/.venv/lib/python3.6/site-packages/requests/utils.py", line 449, in iter_slices
    def iter_slices(string, slice_length):
SystemError: error return without exception set
Exception ignored in: <generator object urlsplit.<locals>.<genexpr> at 0x1095fffc0>
Traceback (most recent call last):
  File "/usr/local/Cellar/python/3.6.4_4/Frameworks/Python.framework/Versions/3.6/lib/python3.6/urllib/parse.py", line 431, in <genexpr>
    if not rest or any(c not in '0123456789' for c in rest):
SystemError: error return without exception set
Exception ignored in: <generator object iter_slices at 0x1095ffba0>
Traceback (most recent call last):
  File "~/.venv/lib/python3.6/site-packages/requests/utils.py", line 449, in iter_slices
    def iter_slices(string, slice_length):
SystemError: error return without exception set
```

It happens when I make requests in many `threading.Thread`

As mentioned in #4367, this is not something that Requests controls. This is almost certianly coming from a library used at some point in the execution of the program (e.g., `json`) that has a C dependency and the threading is leading to this issue. There's nothing for Requests to do as far as we can tell with all of the given information we have.

Have anyone figured out how to fix this issue?

@fhlee74 I seem to have fixed this issue by running my script in a 2.7 env instead of 3.6. The issue did seem to be worse when running > 1 threads in 3.6 but would still happen with just one thread. On 2.7 I have 10 threads running again and haven't seen the exception.

threads are tooooooooooooooooooooooooooooooooooooooooooooooo many ! This is a python's BUG, whether in 2.7 or 3.6 . You can try to use threads pool `from multiprocessing.pool import ThreadPool` . I have 8 threads running and no exception is thrown on 3.6.

this helped me with python 3.7.3 and pycharm https://stackoverflow.com/a/52054289

This worked for me too ^