custom module deadlock import fix

[caronc]

Description:

Related issue (if applicable): #1075

Deadlock issue resolved that was created in #1071

Checklist

• [x] The code change is tested and works locally.
• [x] There is no commented out code in this PR.
• [x] No lint errors (use flake8)
• [x] 100% test coverage

Testing

Anyone can help test this source code as follows:

# Create a virtual environment to work in as follows:
python3 -m venv apprise

# Change into our new directory
cd apprise

# Activate our virtual environment
source bin/activate

# Install the branch
pip install git+https://github.com/caronc/apprise.git@1075-custom-module-deadlock-fix

cat << _EOF > test.py
from apprise.decorators import notify

@notify(on="test")
def my_wrapper(body, title, *args, **kwargs):
  print(f"{title}: {body}")
_EOF

# Test out the changes with the following command:
apprise -t "Test Title" -b "Test Message" \
  -P ./test.py  test://

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.27%. Comparing base (b4798e3) to head (251fa4c).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1077 +/- ## ========================================== - Coverage 99.27% 99.27% -0.01% ========================================== Files 136 136 Lines 17753 17752 -1 Branches 3623 3622 -1 ========================================== - Hits 17625 17624 -1 Misses 119 119 Partials 9 9 ```

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