Fix compilation of fields named 'bytes' or 'str' (#226)

* if you have a field named "bytes" using the bytes type, it doesn't work.
* Enable existing use-case & generalize solution to cover it

Co-authored-by: Spencer <spencer@sf-n.com>

src/betterproto/casing.py

@@ -133,6 +133,16 @@ def lowercase_first(value: str) -> str:
     return value[0:1].lower() + value[1:]
 
 
+def is_reserved_name(value: str) -> bool:
+    if keyword.iskeyword(value):
+        return True
+
+    if value in ("bytes", "str"):
+        return True
+
+    return False
+
+
 def sanitize_name(value: str) -> str:
     # https://www.python.org/dev/peps/pep-0008/#descriptive-naming-styles
-    return f"{value}_" if keyword.iskeyword(value) else value
+    return f"{value}_" if is_reserved_name(value) else value