$ pyenv install 3.5.6
[...]
2019-12-18 16:17:35 (6.33 MB/s) - ‘/home/werner/.pyenv/cache/Python-3.5.6.tar.xz’ saved [15412832/15412832]

Installing Python-3.5.6...

BUILD FAILED (Ubuntu 19.10 using python-build 1.2.15-4-g089b828b)

Inspect or clean up the working tree at /tmp/python-build.20191218161735.29045
Results logged to /tmp/python-build.20191218161735.29122.log

Last 10 log lines:
  File "/tmp/python-build.20191218161735.29045/Python-3.5.6/Lib/runpy.py", line 109, in _get_module_details
    __import__(pkg_name)
  File "/tmp/python-build.20191218161735.29045/Python-3.5.6/Lib/ensurepip/__init__.py", line 5, in <module>
    import tempfile
  File "/tmp/python-build.20191218161735.29045/Python-3.5.6/Lib/tempfile.py", line 45, in <module>
    from random import Random as _Random
  File "/tmp/python-build.20191218161735.29045/Python-3.5.6/Lib/random.py", line 41, in <module>
    from math import log as _log, exp as _exp, pi as _pi, e as _e, ceil as _ceil
ImportError: No module named 'math'
make: *** [Makefile:1048: install] Error 1