Hardening of custom plugin loading by ignore non-Python files

[caronc]

Description:

Related issue (if applicable): here

When Adding Plugin Support to the Apprise API, I learned that the Apprise service can't handle cases where it finds a plugin module that isn't a Python file. It tries to open it and blows up :). This shouldn't happen and has been corrected in this PR.

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• No lint errors (use flake8)
• 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@plugin-module-load-filter-bugfix

# Test out the changes with the following command:
apprise -t "Test Title" -b "Test Message" \
  <apprise url related to ticket>

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (2057107) 100.00% compared to head (6a824f3) 100.00%.

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Additional details and impacted files

@@            Coverage Diff            @@
##            master      #853   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          118       118           
  Lines        15366     15373    +7     
  Branches      3153      3154    +1     
=========================================
+ Hits         15366     15373    +7     

Impacted Files	Coverage Δ
apprise/utils.py	100.00% <100.00%> (ø)

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