Odd behaviour of snake_case functions

Describe the bug
The names of apis are odd, having '_' in irregular and unpredictable places

To Reproduce
The following behavior can be observed using the snake_case function from utils:

utils.snake_case("get_v2_abc")
'get_v2_abc'
utils.snake_case("get_v2_abc_deFgHij")
'get_v_2_abc_de_fg_hij'

These would for example be created for paths: /v2/abc and /v2/abc/deFgHij

The results however inserts an additional underscore between v and 2, so v_2 instead of v2.

Expected behavior
results should be:
get_v2_abc_de_fg_hij

Desktop (please complete the following information):

• OS: CentOS7
• Python Version: 3.8
• openapi-python-client version: github-master from today

Suggestion:
snake_case uses split_words function , which only splits on Aa, so a capital letter followed by a lowercase one, not on underscore. split_words should first split on underscore, then split on capital followed by lowercase. This would solve the issue of mixed snake and camelcase names.

Some additional odd outcomes:

>>> utils.snake_case("MODEL4dd")
'model4_dd'
>>> utils.snake_case("MODEl4dd")
'mod_el_4_dd'

These of course violate camelCase, but in my estimate should all be treated as one word, i.e. multiple consecutive capital letters are treated as a single block. So above model4dd would be the outcome in both cases.

Beginne damit, die Implementierungen von utils.snake_case und split_words im Python-Generator zu finden. Reproduziere die Beispiele aus dem Issue und überprüfe anschließend, dass gemischte snake_case- und camelCase-Eingaben die erwarteten Wörter erzeugen und dass die MODEL4dd-Fälle ein Wort bleiben; füge die relevanten Utility-Tests hinzu oder führe sie aus, falls vorhanden.