Pants is a multi-library python project and dogfoods itself for its own builds:

$ ./pants cloc src/:: tests/:: contrib/:: 2>/dev/null
<http://github.com/AlDanial/cloc|github.com/AlDanial/cloc> v 1.80  T=3.80 s (476.5 files/s, 65468.5 lines/s)
-----------------------------------------------------------------------------------
Language                         files          blank        comment           code
-----------------------------------------------------------------------------------
Python                            1173          30551          30644         101112
Rust                                84           5089           4199          38406
Java                               177           1860           2039           8850
Markdown                            84           1953              0           5227
JSON                                41             28              0           4359
Scala                               42            458            540           2183
Protocol Buffers                    23            477           2253           1826
Mustache                            32            124             21           1430
Bourne Shell                        22            269            163           1180
CSS                                  4            114             52            832
JavaScript                          52            105             67            644
TOML                                27             45             19            523
XML                                 29             22             36            468
INI                                  2             91              0            411
Go                                  12             29              9            138
C++                                  3              8              0             31
YAML                                 1              4              6             26
Windows Resource File                1              8              0             23
Bourne Again Shell                   2              2              0             10
C/C++ Header                         1              4              0             10
-----------------------------------------------------------------------------------
SUM:                              1812          41241          40048         167689
-----------------------------------------------------------------------------------

It's a fairly complex project though; so - a bit ironically - it is probably not the best example of how you might set your project up. You might start looking here though while reading through https://www.pantsbuild.org/python_readme.html and https://www.pantsbuild.org/3rdparty_py.html.