title: "RuntimeError: dictionary keys changed during iteration" when attempting to call add_attachment for Jira issue object, but dictionaries aren't used?
body: <p>I am attempting to just add a csv file to my issues as a test, but I keep receiving the error:</p>
  
  <pre><code>RuntimeError: dictionary keys changed during iteration
  </code></pre>
  
  <p>Here is the code (I've removed the parameters for server, username and password):</p>
  
  <pre><code>from jira import JIRA
  
  options = {"server": "serverlinkgoeshere"}
  jira = JIRA(options, basic_auth=('username', 'password'))
  issuesList = jira.search_issues(jql_str='', startAt=0, maxResults=100)
  for issue in issuesList:
      with open("./csv/Adobe.csv",'rb') as f:
          jira.add_attachment(issue=issue, attachment=f)
      f.close()
  </code></pre>
  
  <p>I'm at a loss, I'm not changing any dictionary keys in my code. Here is the full error message:</p>
  
  <pre><code>Traceback (most recent call last):
    File "C:/Users/USER/PycharmProjects/extractor/main/jiraCSVDupdate.py", line 8, in &lt;module&gt;
      jira.add_attachment(issue=issue, attachment=f)
    File "C:\Users\USER\AppData\Roaming\Python\Python38\site-packages\jira\client.py", line 126, in wrapper
      result = func(*arg_list, **kwargs)
    File "C:\Users\USER\AppData\Roaming\Python\Python38\site-packages\jira\client.py", line 787, in add_attachment
      url, data=m, headers=CaseInsensitiveDict({'content-type': m.content_type, 'X-Atlassian-Token': 'nocheck'}), retry_data=file_stream)
    File "C:\Users\USER\AppData\Roaming\Python\Python38\site-packages\jira\utils\__init__.py", line 41, in __init__
      for key, value in super(CaseInsensitiveDict, self).items():
  RuntimeError: dictionary keys changed during iteration
  </code></pre>
  
  <p>References:
  Jira add_attachment example:
  <a href="https://jira.readthedocs.io/en/master/examples.html#attachments" rel="nofollow noreferrer">https://jira.readthedocs.io/en/master/examples.html#attachments</a></p>
  
  <p>add_attachment source code:
  <a href="https://jira.readthedocs.io/en/master/_modules/jira/client.html#JIRA.add_attachment" rel="nofollow noreferrer">https://jira.readthedocs.io/en/master/_modules/jira/client.html#JIRA.add_attachment</a></p>