Assuming a byte has 8 bits, we have 8 ** 2 = 64.
Thus, odd numbers of bytes would have needed padding.
In fact, base64 works directly on bits, not assuming an 8-bit clean environment.
For bits, we have 2 ** 6 = 64.
Thus, bytes are grouped by 3 (lcm(6, 8) / 8 = 18 / 8 = 3).
When encoding a 1-byte or 2-byte group, it is padded with 0.
In encoded output, a final ==/= sequence indicates the last group contains only 1/2 bytes.