Synthesis failed for cloudbilling

Hello! Autosynth couldn't regenerate cloudbilling. :broken_heart:
Here's the output from running synth.py:
Cloning into 'working_repo'...
Checking out files:  48% (18353/37616)   
Checking out files:  49% (18432/37616)   
Checking out files:  50% (18808/37616)   
Checking out files:  51% (19185/37616)   
Checking out files:  52% (19561/37616)   
Checking out files:  53% (19937/37616)   
Checking out files:  54% (20313/37616)   
Checking out files:  55% (20689/37616)   
Checking out files:  56% (21065/37616)   
Checking out files:  57% (21442/37616)   
Checking out files:  58% (21818/37616)   
Checking out files:  59% (22194/37616)   
Checking out files:  60% (22570/37616)   
Checking out files:  61% (22946/37616)   
Checking out files:  62% (23322/37616)   
Checking out files:  63% (23699/37616)   
Checking out files:  64% (24075/37616)   
Checking out files:  65% (24451/37616)   
Checking out files:  66% (24827/37616)   
Checking out files:  67% (25203/37616)   
Checking out files:  68% (25579/37616)   
Checking out files:  69% (25956/37616)   
Checking out files:  70% (26332/37616)   
Checking out files:  71% (26708/37616)   
Checking out files:  72% (27084/37616)   
Checking out files:  73% (27460/37616)   
Checking out files:  74% (27836/37616)   
Checking out files:  75% (28212/37616)   
Checking out files:  76% (28589/37616)   
Checking out files:  77% (28965/37616)   
Checking out files:  78% (29341/37616)   
Checking out files:  79% (29717/37616)   
Checking out files:  80% (30093/37616)   
Checking out files:  81% (30469/37616)   
Checking out files:  82% (30846/37616)   
Checking out files:  83% (31222/37616)   
Checking out files:  84% (31598/37616)   
Checking out files:  85% (31974/37616)   
Checking out files:  86% (32350/37616)   
Checking out files:  87% (32726/37616)   
Checking out files:  88% (33103/37616)   
Checking out files:  89% (33479/37616)   
Checking out files:  90% (33855/37616)   
Checking out files:  91% (34231/37616)   
Checking out files:  92% (34607/37616)   
Checking out files:  93% (34983/37616)   
Checking out files:  94% (35360/37616)   
Checking out files:  95% (35736/37616)   
Checking out files:  96% (36112/37616)   
Checking out files:  97% (36488/37616)   
Checking out files:  98% (36864/37616)   
Checking out files:  98% (36960/37616)   
Checking out files:  99% (37240/37616)   
Checking out files: 100% (37616/37616)   
Checking out files: 100% (37616/37616), done.
Switched to branch 'autosynth-cloudbilling'
Running synthtool
['/tmpfs/src/git/autosynth/env/bin/python3', '-m', 'synthtool', 'synth.py', '--']
synthtool > Executing /tmpfs/src/git/autosynth/working_repo/synth.py.
Traceback (most recent call last):
  File "/home/kbuilder/.pyenv/versions/3.6.1/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/home/kbuilder/.pyenv/versions/3.6.1/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/tmpfs/src/git/autosynth/env/lib/python3.6/site-packages/synthtool/__main__.py", line 87, in <module>
    main()
  File "/tmpfs/src/git/autosynth/env/lib/python3.6/site-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/tmpfs/src/git/autosynth/env/lib/python3.6/site-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/tmpfs/src/git/autosynth/env/lib/python3.6/site-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/tmpfs/src/git/autosynth/env/lib/python3.6/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/tmpfs/src/git/autosynth/env/lib/python3.6/site-packages/synthtool/__main__.py", line 79, in main
    spec.loader.exec_module(synth_module)  # type: ignore
  File "<frozen importlib._bootstrap_external>", line 678, in exec_module
  File "<frozen importlib._bootstrap>", line 205, in _call_with_frames_removed
  File "/tmpfs/src/git/autosynth/working_repo/synth.py", line 26, in <module>
    from lxml import etree
ModuleNotFoundError: No module named 'lxml'
synthtool > Wrote metadata to synth.metadata.

Synthesis failed
Google internal developers can see the full log here.
googleapis / google-api-java-client-services

Synthesis failed for cloudbilling #1756
