Python: add support for __about__.py

[fredrikaverpil]

Is your feature request related to a problem? Please describe.

I cannot make release-please bump the version string inside any other Python file than __init__.py, as this is today hard-coded:

release-please/src/strategies/python.ts

Lines 105 to 120 in 52594b1

	if (!projectName) {
	this.logger.warn('No project/component found.');
	} else {
	[projectName, projectName.replace(/-/g, '_')]
	.flatMap(packageName => [
	`${packageName}/__init__.py`,
	`src/${packageName}/__init__.py`,
	])
	.forEach(packagePath =>
	updates.push({
	path: this.addPath(packagePath),
	createIfMissing: false,
	updater: new PythonFileWithVersion({version}),
	})
	);
	}

Some Python projects doesn't use __init__.py to store the version string. Some projects like to place this in a __about__.py file or __version__.py file instead. In short, this separates concerns so that the version string can be watched outside of any other potential code changes. In my case, I wanted release-please to bump an __about__.py file.

Describe the solution you'd like

• I'd like to have release-please attempt to bump __init__.py like it does today for backwards compatibility reasons.
• I'd like to define the desired filename in which the version string will be bumped, using the version-file manifest configuration (but this is currently not taken into consideration for release-please's Python strategy).

Describe alternatives you've considered

N/A

PR proposal

This enables the use of version-file: #2434