Update supported Python classifiers

[jenstroeger]

Description

Considering that

commitizen/pyproject.toml

Lines 47 to 48 in 2ff9f15

	[tool.poetry.dependencies]
	python = "^3.6.2"

the current list of classifiers

commitizen/pyproject.toml

Lines 42 to 44 in 2ff9f15

	"Programming Language :: Python :: 3.6",
	"Programming Language :: Python :: 3.7",
	"Programming Language :: Python :: Implementation :: CPython",

are incomplete. Should probably be fixed as they’re used by other services including PyPI.

Steps to reproduce

Read the code 😉

Current behavior

n/a

Desired behavior

n/a

Screenshots

No response

Environment

Package metadata.

[woile]

Thanks! Feel free to submit a PR for this 💪🏻