Errors when parsing singularity definitions with comments before bootstrap line

[StopMotionCuber]

Expected Behavior

When parsing a singularity definition with comments before the bootstrap line, the file should be successfully parsed

Actual Behavior

The parsing throws an UnboundLocalError: local variable 'comments' referenced before assignment

Steps to Reproduce

1. Create a definition file with the following content:

# Minimal Example - This comment line is important for reproduction purposes

Bootstrap: docker
From: ubuntu:16.04

2. Start a python interpreter in that directory, with spython installed
3. Execute

from spython.main.parse.parsers import SingularityParser
SingularityParser("minimal.def")

in that directory

Context

• Operating System: Arch Linux
• singularity version: not installed
• spython version: 0.1.15
• python version: 3.9.7

Failure Logs

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "[redacted]/venv/lib/python3.9/site-packages/spython/main/parse/parsers/singularity.py", line 30, in __init__
    super(SingularityParser, self).__init__(filename, load)
  File "[redacted]/venv/lib/python3.9/site-packages/spython/main/parse/parsers/base.py", line 56, in __init__
    self.parse()
  File "[redacted]/venv/lib/python3.9/site-packages/spython/main/parse/parsers/singularity.py", line 41, in parse
    self.load_recipe()
  File "[redacted]/venv/lib/python3.9/site-packages/spython/main/parse/parsers/singularity.py", line 333, in load_recipe
    elif stripped.startswith("#") and stripped not in comments:
UnboundLocalError: local variable 'comments' referenced before assignment

Possible Fix

https://github.com/singularityhub/singularity-cli/blob/master/spython/main/parse/parsers/singularity.py#L318
That comments definition could to be repeated somewhere above

[vsoch]

Oh this is easy to fix! I'll get a PR in for you at the end of the work day. Thanks for pointing out the issue!

[vsoch]

@StopMotionCuber would you care to review? #181