identifier() should quote if uppercase characters

Previously the identifier() function wouldn't quote the identifier
solely because it contained uppercase characters. The new behaviour of
quoting if there are any uppercase characters is intended to be more in
line with what people expect. It's also easier to recreate the old
behaviour by calling lower() on the input beforehand, than it is to try
and create the new behaviour with the old function.

src/pg8000/converters.py

@@ -772,17 +772,21 @@ def make_params(py_types, values):
     return tuple([make_param(py_types, v) for v in values])
 
 
+def _quote_letter(c):
+    return c.isupper() if c.isalpha() else True
+
+
 def identifier(sql):
     if not isinstance(sql, str):
         raise InterfaceError("identifier must be a str")
 
     if len(sql) == 0:
         raise InterfaceError("identifier must be > 0 characters in length")
 
-    quote = not sql[0].isalpha()
+    quote = _quote_letter(sql[0])
 
     for c in sql[1:]:
-        if not (c.isalpha() or c.isdecimal() or c in "_$"):
+        if _quote_letter(c) and c not in "0123456789_$":
             if c == "\u0000":
                 raise InterfaceError(
                     "identifier cannot contain the code zero character"

test/test_converters.py

@@ -345,7 +345,11 @@ def test_identifier_quoted_null():
         (" Table", '" Table"'),
         ("A Table", '"A Table"'),
         ('A " Table', '"A "" Table"'),
-        ("Table$", "Table$"),
+        ("table$", "table$"),
+        ("Table$", '"Table$"'),
+        ("tableఐ", "tableఐ"),  # Unicode character 0C10 which is uncased
+        ("table", "table"),
+        ("tAble", '"tAble"'),
     ],
 )
 def test_identifier_success(value, expected):