Description of problem:
If a process disappears while iterating the loop in pidstats.reload_threads(), we get a RuntimeError as shown below. This is because we cannot remove an entry from a dictionary while iterating the dictionary.

This problem is the cause of the following Tuned error reported in bug#1666562:
2019-01-16 03:22:34,859 ERROR    tuned.units.manager: BUG: Unhandled exception in start_tuning: dictionary changed size during iteration

Version-Release number of selected component (if applicable):
python3-linux-procfs-0.6-5.el8.noarch

How reproducible:
always

Steps to Reproduce:
1. Add the following line to the beginning of pidstats.reload_threads() in /usr/lib/python3.6/site-packages/procfs/procfs.py:
import pdb; pdb.set_trace()
2. Start some process
3. Start the python interpreter and proceed as follows:
# python3
Python 3.6.8 (default, Jan 11 2019, 02:17:16) 
[GCC 8.2.1 20180905 (Red Hat 8.2.1-3)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import procfs
>>> ps = procfs.pidstats()
>>> ps.reload_threads()
> /usr/lib/python3.6/site-packages/procfs/procfs.py(463)reload_threads()
-> for pid in self.processes.keys():
(Pdb) next
> /usr/lib/python3.6/site-packages/procfs/procfs.py(464)reload_threads()
-> try:
(Pdb) 

At this point, terminate the process started in step 2. Return to the
python interpreter:

(Pdb) continue
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.6/site-packages/procfs/procfs.py", line 465, in reload_threads
    self.processes[pid].load_threads()
RuntimeError: dictionary changed size during iteration


Actual results:
RuntimeError is raised

Expected results:
reload_threads() finishes without errors

Additional info:
A fix for the problem has been sent upstream:
https://lists.fedorahosted.org/archives/list/tuna-devel@lists.fedorahosted.org/thread/PK354CZETOV23PJPMJ7CZCK7SOOV2JX6/