title: Getting AttributeError: module 'base64' has no attribute 'decodestring' error while running on python 3.9.6
body: <p>Issue description:</p>
  <p>Getting <code>AttributeError: module 'base64' has no attribute 'decodestring'</code> error while running on python 3.9.6</p>
  <p>Steps to reproduce:</p>
  <p>Below is a dummy program, while running on python 3.9.6, I am getting `AttributeError: module 'base64' has no attribute 'decodestring'`` error:</p>
  <pre><code>from ldif3 import LDIFParser
  
  parser = LDIFParser(open('dse3.ldif', 'rb'))
  for dn, entry in parser.parse():
      if dn == &quot;cn=Schema Compatibility,cn=plugins,cn=config&quot;:
          if entry['nsslapd-pluginEnabled'] == ['on']:
              print('Entry record: %s' % dn)
  </code></pre>
  <p>Error message:</p>
  <pre><code>python3.9 1.py                              ✔    venvpy3.9   11:12:01 
  Traceback (most recent call last):
    File &quot;/Users/rasrivas/local_test/1.py&quot;, line 4, in &lt;module&gt;
      for dn, entry in parser.parse():
    File &quot;/Users/rasrivas/local_test/venvpy3.9/lib/python3.9/site-packages/ldif3.py&quot;, line 384, in parse
      yield self._parse_entry_record(block)
    File &quot;/Users/rasrivas/local_test/venvpy3.9/lib/python3.9/site-packages/ldif3.py&quot;, line 357, in _parse_entry_record
      attr_type, attr_value = self._parse_attr(line)
    File &quot;/Users/rasrivas/local_test/venvpy3.9/lib/python3.9/site-packages/ldif3.py&quot;, line 315, in _parse_attr
      attr_value = base64.decodestring(line[colon_pos + 2:])
  AttributeError: module 'base64' has no attribute 'decodestring'
  </code></pre>
  <p>python version</p>
  <pre><code>python --version 
  Python 3.9.6
  </code></pre>
  <p>Operating system:</p>
  <p>macOS 11.5.2</p>
  <p>Python version:</p>
  <pre><code>python --version 
  Python 3.9.6
  </code></pre>
  <p>python-ldap version:</p>
  <p><code>ldif3-3.2.2</code></p>