iterating over DTD file including external entities

[Adam Byer]

Hi. New user here. Apologies if this is not the right place. I'm loading a DTD and iterating over the elements like so...

```
from lxml import etree

with open(dtd_file) as f:
    dtd_content = f.read()

dtd = etree.DTD(StringIO(dtd_content))

for el in dtd.iterelements():
...

The DTD file (dtd_file) has a reference to an external entity like this...
```
<!ENTITY % emp SYSTEM "other_file.dtd">
```

It doesn't seem that `iterelements` includes the elements from the external entity (other_file.dtd). Is it possible to have those included?

Thank you.

Python : sys.version_info(major=3, minor=9, micro=10, releaselevel='final', serial=0)
lxml.etree : (4, 9, 2, 0)
libxml used : (2, 9, 13)
libxml compiled : (2, 9, 13)
libxslt used : (1, 1, 35)
libxslt compiled : (1, 1, 35)