Update strum version ranges
It appears to only be used for #[strum(serialize = "XXXX")] which does
not seem likely to break anything per the changelog:
@@ -44,8 +44,8 @@ fxhash = "0.2"
statrs = ">= 0.11, <= 0.13"
bio-types = ">=0.5.1"
fnv = "1.0"
-strum = ">= 0.16, <= 0.18"
-strum_macros = ">= 0.16, <= 0.18"
+strum = ">= 0.16, <= 0.20"
+strum_macros = ">= 0.16, <= 0.20"
getset = "0.0.9"
enum-map = "0.6"
triple_accel = "0.3"
