metadata: auto-convert YAML special float values: .nan .inf -.inf

Even for people who know what the special floats not-a-number, infinity,
and negative infinity, they don't necessarily know the YAML 1.2 syntax for
these.  I didn't.  And I've spent some quality time fighting things with
those values.  They are also easy to reliably convert to string values.

fdroidserver/metadata.py

@@ -20,6 +20,7 @@
 
 import git
 from pathlib import Path
+import math
 import platform
 import os
 import re
@@ -897,6 +898,14 @@ def _normalize_type_string(v):
         if v:
             return 'true'
         return 'false'
+    if isinstance(v, float):
+        # YAML 1.2 values for NaN, Inf, and -Inf
+        if math.isnan(v):
+            return '.nan'
+        if math.isinf(v):
+            if v > 0:
+                return '.inf'
+            return '-.inf'
     return str(v)