当我们通过
Storyboard布局UILabel时，可以清楚的调整字体的粗细选项，如图：
但是代码并未明确有这么多的选项，那怎么办呢？
label.font = UIFont.systemFont(ofSize: 17, weight: UIFontWeightMedium)
@available(iOS 8.2, *)
public let UIFontWeightUltraLight: CGFloat
@available(iOS 8.2, *)
public let UIFontWeightThin: CGFloat
@available(iOS 8.2, *)
public let UIFontWeightLight: CGFloat
@available(iOS 8.2, *)
public let UIFontWeightRegular: CGFloat
@available(iOS 8.2, *)
public let UIFontWeightMedium: CGFloat
@available(iOS 8.2, *)
public let UIFontWeightSemibold: CGFloat
@available(iOS 8.2, *)
public let UIFontWeightBold: CGFloat
@available(iOS 8.2, *)
public let UIFontWeightHeavy: CGFloat
@available(iOS 8.2, *)
public let UIFontWeightBlack: CGFloat
