pgn: accept tag pairs with leading whitespace (fixes #1115)

The PGN standard specifies that whitespace is not significant.
`read_game()` was checking `line.startswith("[")` and matching
`TAG_REGEX` against the raw line, both of which fail when a tag pair
is preceded by horizontal whitespace (e.g. a file that begins with
" [Event ...]").

Strip leading whitespace before the `startswith` guard and the regex
match so that indented tag pairs are recognised correctly.

Author: gaoflow

chess/pgn.py

@@ -1636,13 +1636,14 @@ def read_game(handle: TextIO, *, Visitor: Any = GameBuilder) -> Any:
                 if not isinstance(managed_headers, Headers):
                     unmanaged_headers = Headers({})
 
-        if not line.startswith("["):
+        stripped = line.lstrip()
+        if not stripped.startswith("["):
             break
 
         consecutive_empty_lines = 0
 
         if not skipping_game:
-            tag_match = TAG_REGEX.match(line)
+            tag_match = TAG_REGEX.match(stripped)
             if tag_match:
                 visitor.visit_header(tag_match.group(1), tag_match.group(2))
                 if unmanaged_headers is not None: