with open('log.log', 'rt', encoding='utf-8') as file:
    buf = []
    result = []
    for line in file:
        line = line.rstrip()
        if len(buf) and line.startswith('Warning'):
            result.append(' '.join(buf))
            buf = []
        buf.append(line)

    if len(buf):
        result.append(' '.join(buf))

print(result)