AMF exporter issue?

Snippet:

```
import cadquery as cq
from cadquery import Vector
from cadquery import exporters

wp = cq.Workplane('front')

if True:
# --- simple union example ---
sp1 = wp.sphere(radius=10).translate(Vector(-5, 0, 0))
sp2 = wp.sphere(radius=8).translate(Vector(5, 0, 0))

scene = sp1.union(toUnion=sp2)

with open('try.amf', 'w') as f:
f.write(exporters.toString(sp1, 'AMF', 0.01))
```

Problem (line numbers are not in sync with the snippet lines):
```
Traceback (most recent call last):
File "/home/bobajfej/Prg/CQModels/try.py", line 19, in
f.write(exporters.toString(sp1, 'AMF', 0.01))
File "/home/bobajfej/Prg/CQModels/env.conda/lib/python3.8/site-packages/cadquery/utils.py", line 36, in wrapped
return f(*args, **kwargs)
File "/home/bobajfej/Prg/CQModels/env.conda/lib/python3.8/site-packages/cadquery/occ_impl/exporters/__init__.py", line 115, in toString
exportShape(shape, exportType, s, tolerance, angularTolerance)
File "/home/bobajfej/Prg/CQModels/env.conda/lib/python3.8/site-packages/cadquery/utils.py", line 36, in wrapped
return f(*args, **kwargs)
File "/home/bobajfej/Prg/CQModels/env.conda/lib/python3.8/site-packages/cadquery/occ_impl/exporters/__init__.py", line 167, in exportShape
aw.writeAmf(fileLike)
File "/home/bobajfej/Prg/CQModels/env.conda/lib/python3.8/site-packages/cadquery/occ_impl/exporters/amf.py", line 39, in writeAmf
amf = ET.ElementTree(amf).write(outFile, xml_declaration=True)
File "/home/bobajfej/Prg/CQModels/env.conda/lib/python3.8/xml/etree/ElementTree.py", line 772, in write
serialize(write, self._root, qnames, namespaces,
File "/home/bobajfej/Prg/CQModels/env.conda/lib/python3.8/xml/etree/ElementTree.py", line 937, in _serialize_xml
_serialize_xml(write, e, qnames, None,
File "/home/bobajfej/Prg/CQModels/env.conda/lib/python3.8/xml/etree/ElementTree.py", line 937, in _serialize_xml
_serialize_xml(write, e, qnames, None,
File "/home/bobajfej/Prg/CQModels/env.conda/lib/python3.8/xml/etree/ElementTree.py", line 937, in _serialize_xml
_serialize_xml(write, e, qnames, None,
[Previous line repeated 2 more times]
File "/home/bobajfej/Prg/CQModels/env.conda/lib/python3.8/xml/etree/ElementTree.py", line 912, in _serialize_xml
write("<" + tag)
TypeError: string argument expected, got 'bytes'

```

Version: `cadquery 2.1RC1 py3.8 cadquery`