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

[gaoflow]

Summary

read_game() (and therefore read_headers()) failed to parse game
headers that are preceded by horizontal whitespace, e.g. a PGN string
that begins with " [Event ...]".

The PGN standard (section 2.1) specifies that white space is not
significant, so a tag pair indented with spaces or tabs should be
accepted.

Root cause

Two checks in the header-parsing loop used the raw line string:

1. if not line.startswith("["): — breaks out of the header loop for any
   line that doesn't begin immediately with [.
2. TAG_REGEX.match(line) — the regex is anchored at ^ and will not
   match a line with leading whitespace.

Together these caused the parser to treat an indented first tag as the
start of the movetext section, so no headers were ever parsed and the
game was returned with default header values.

Fix

Strip leading whitespace into a local stripped variable before the
startswith guard and the regex match. The original line value is
preserved for the movetext tokeniser that runs after the header loop.

Reproducer (from #1115)

import chess.pgn, io

pgn = io.StringIO(' [Event "Test"]\n[White "Alice"]\n[Black "Bob"]\n[Result "*"]\n\n1. e4 e5 *')
game = chess.pgn.read_game(pgn)
assert game.headers["Event"] == "Test"   # previously returned "?"
assert game.headers["White"] == "Alice"  # previously returned "?"

This pull request was prepared with the assistance of AI, under my direction and review.