As suggested by Serhiy, this changes the C implementation of TreeBuilder.start to remove the default value.

Note however, that the C implementation of TreeBuilder.start still accepts None as a value whereas the Python implementation TypeErrors (see snippet at end).

I looked into changing the C implementation to also throw a TypeError. Unfortunately, it appears treebuilder_handle_start is called elsewhere where attrib can be Py_None (set on line 3310) and I'm not familiar enough with this module to know if I can break that:

~/dev/cpython bpo39495 λ ./python.exe                       
Python 3.9.0a3+ (heads/bpo39495_2-dirty:2e6569b669, Jan 29 2020, 22:45:33) 
[Clang 11.0.0 (clang-1100.0.33.17)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import xml.etree.ElementTree
>>> # show the fix
>>> xml.etree.ElementTree.TreeBuilder().start("asdf")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: start expected 2 arguments, got 1
>>> # function still accepts None
>>> xml.etree.ElementTree.TreeBuilder().start("asdf", None)
<Element 'asdf' at 0x10b2e6710>

vs

~ λ python3.9
Python 3.9.0a3+ (heads/master:188bb5b, Jan 29 2020, 22:55:03) 
[Clang 11.0.0 (clang-1100.0.33.17)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import xml.etree.ElementTree
>>> # not fixed
>>> xml.etree.ElementTree.TreeBuilder().start("asdf")
<Element 'asdf' at 0x10b89f2c0>
>>> # function accepts None
>>> xml.etree.ElementTree.TreeBuilder().start("asdf", None)
<Element 'asdf' at 0x10b89f310>
>>> from test.support import import_fresh_module  
>>> pyElementTree = import_fresh_module('xml.etree.ElementTree', blocked=['_elementtree'])
>>> # show the inconsistency with python implementation
>>> pyElementTree.TreeBuilder().start("asdf")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: start() missing 1 required positional argument: 'attrs'
>>> # function does not accept None, throws TypeError
>>> pyElementTree.TreeBuilder().start("asdf", None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/shantanu/.pyenv/versions/3.9-dev/lib/python3.9/xml/etree/ElementTree.py", line 1463, in start
    self._last = elem = self._factory(tag, attrs)
  File "/Users/shantanu/.pyenv/versions/3.9-dev/lib/python3.9/xml/etree/ElementTree.py", line 171, in __init__
    raise TypeError("attrib must be dict, not %s" % (
TypeError: attrib must be dict, not NoneType