[MNT] 0.18.0 change action - numba as soft dependency (#3843)

This PR turns `numba` into a soft dependency, as scheduled for 0.18.0.

Relies on #3631 which isolates
`numba`, which was merged pre-0.17.0 to prepare the change.

pyproject.toml

@@ -45,7 +45,6 @@ classifiers = [
 requires-python = ">=3.7,<3.12"
 dependencies = [
     "deprecated>=1.2.13",
-    "numba>=0.53; python_version < '3.11'",
     "numpy>=1.21.0,<1.25",
     "pandas>=1.1.0,<2.0.0",
     "scikit-learn>=0.24.0,<1.3.0",
@@ -68,6 +67,7 @@ all_extras = [
     "kotsu>=0.3.1",
     "matplotlib>=3.3.2",
     "mne",
+    "numba>=0.53; python_version < '3.11'",
     "pmdarima>=1.8.0,!=1.8.1,<3.0.0",
     "prophet>=1.1",
     "pykalman>=0.9.5; python_version < '3.11'",