Assembler fails to interpret RTS mnemonics

[craigthomas]

Describe the bug
Assembling RTS statements are skipped in some instances.

To Reproduce
Steps to reproduce the behavior:

1. Create the following file called rtsbug.asm:

                NAM     RTSBUG
                ORG     $0E00
START           LDA     #$01            ;
                RTS                     ;
                END     START

2. Assemble the listing with the following command:

python3 assembler.py --print rtsbug.asm

3. The RTS statement will not be compiled:

-- Assembled Statements --
$0000                         NAM RTSBUG                         ;                                         
$0E00                         ORG $0E00                          ;                                         
$0E00 8601            START   LDA #$01                           ;                                         
$0E02                         RTS ;                              ;                                         
$0E02                         END START                          ;  

Expected behavior
Expected behavior results in the following output:

-- Assembled Statements --
$0000                         NAM RTSBUG                         ;                                         
$0E00                         ORG $0E00                          ;                                         
$0E00 8601            START   LDA #$01                           ;                                         
$0E02 39                      RTS ;                              ;                                         
$0E03                         END START                          ;  

Desktop (please complete the following information):

• OS: Ubuntu 20.04
• Python Version: 3.8.10