Handle trailing build metadata in Python version string

[andrewjcg]

Don't fail Python versions when the version string contains trailing build metadata after the + (as per: https://semver.org/#spec-item-10).

[benfred]

Can you add a test case that exercises this functionality? Something like

py-spy/src/version.rs

Lines 132 to 143 in 75adc45

	// v2.7.15+ is a valid version string apparently: https://github.com/benfred/py-spy/issues/81
	let version = Version::scan_bytes(b"2.7.15+ (default, Oct 2 2018, 22:12:08)").unwrap();
	assert_eq!(
	version,
	Version {
	major: 2,
	minor: 7,
	patch: 15,
	release_flags: "".to_owned()
	}
	);
	}

[andrewjcg]

Added test and also updated regex to match full build metadata format supported by https://semver.org/#spec-item-10.