问题描述：
在Python 3.9.6上运行时出现AttributeError: module 'base64' has no attribute 'decodestring'错误
复制步骤：
下面是一个虚拟程序，在python 3.9.6上运行时，我得到了AttributeError：模块'base64'没有属性'decodestring'错误：

from ldif3 import LDIFParser

parser = LDIFParser(open('dse3.ldif', 'rb'))
for dn, entry in parser.parse():
    if dn == "cn=Schema Compatibility,cn=plugins,cn=config":
        if entry['nsslapd-pluginEnabled'] == ['on']:
            print('Entry record: %s' % dn)

错误信息：

python3.9 1.py                              ✔    venvpy3.9   11:12:01 
Traceback (most recent call last):
  File "/Users/rasrivas/local_test/1.py", line 4, in <module>
    for dn, entry in parser.parse():
  File "/Users/rasrivas/local_test/venvpy3.9/lib/python3.9/site-packages/ldif3.py", line 384, in parse
    yield self._parse_entry_record(block)
  File "/Users/rasrivas/local_test/venvpy3.9/lib/python3.9/site-packages/ldif3.py", line 357, in _parse_entry_record
    attr_type, attr_value = self._parse_attr(line)
  File "/Users/rasrivas/local_test/venvpy3.9/lib/python3.9/site-packages/ldif3.py", line 315, in _parse_attr
    attr_value = base64.decodestring(line[colon_pos + 2:])
AttributeError: module 'base64' has no attribute 'decodestring'

python版本

python --version 
Python 3.9.6

操作系统：
macOS 11.5.2
Python版本：

python --version 
Python 3.9.6

python-ldap版本：
ldif3-3.2.2