The C accelerated version of `xml.etree.ElementTree.TreeBuilder.start` has a default value for `attrs`, whereas the pure Python version does not.

```
In [41]: sys.version                                                                                                               
Out[41]: '3.8.1 (default, Jan 23 2020, 23:36:06) \n[Clang 11.0.0 (clang-1100.0.33.17)]'

In [42]: import xml.etree.ElementTree                                                                                              

In [43]: inspect.signature(xml.etree.ElementTree.TreeBuilder.start)                                                                
Out[43]: <Signature (self, tag, attrs=None, /)>

In [44]: from test.support import import_fresh_module                                                                              

In [45]: pyElementTree = import_fresh_module('xml.etree.ElementTree', blocked=['_elementtree'])                                    

In [46]: inspect.signature(pyElementTree.TreeBuilder.start)                                                                        
Out[46]: <Signature (self, tag, attrs)>
```

From PEP 399 (https://www.python.org/dev/peps/pep-0399/)
```
Acting as a drop-in replacement also dictates that no public API be provided in accelerated code that does not exist in the pure Python code. Without this requirement people could accidentally come to rely on a detail in the accelerated code which is not made available to other VMs that use the pure Python implementation. 
```

Based on https://github.com/python/cpython/blob/master/Modules/_elementtree.c#L2700 and the behaviour at runtime, something like the following patch could work:
```
diff --git a/Lib/xml/etree/ElementTree.py b/Lib/xml/etree/ElementTree.py
index c8d898f328..bbfc1afe08 100644
--- a/Lib/xml/etree/ElementTree.py
+++ b/Lib/xml/etree/ElementTree.py
@@ -1452,7 +1452,7 @@ class TreeBuilder:
         """Add text to current element."""
         self._data.append(data)
 
-    def start(self, tag, attrs):
+    def start(self, tag, attrs=None):
         """Open new element and return it.
 
         *tag* is the element name, *attrs* is a dict containing element
@@ -1460,6 +1460,8 @@ class TreeBuilder:
 
         """
         self._flush()
+        if attrs is None:
+            attrs = {}
         self._last = elem = self._factory(tag, attrs)
         if self._elem:
             self._elem[-1].append(elem)
```

Happy to submit a PR!

According to the documentation the attrs parameter does not have default value. I think that the C implementation should be changed.

https://docs.python.org/3/library/xml.etree.elementtree.html#xml.etree.ElementTree.TreeBuilder.start

New changeset 4edc95cf0a2960431621eee9bc194f6225f1690b by Shantanu in branch 'master':
bpo-39495: Remove default value from C impl of TreeBuilder.start (GH-18275)
https://github.com/python/cpython/commit/4edc95cf0a2960431621eee9bc194f6225f1690b