Slack files upload deprecation

[delenamalan]

Description:

Related issue (if applicable): #1126

The Slack files.upload method is deprecated. Instead, you now need to use the files.getUploadURLExternal and files.completeUploadExternal methods to upload files to Slack.

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.

Local test result:

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/delenamalan/apprise.git@slack-files-upload-deprecation

# Test out the changes with the following command:
apprise -t "Test Title" -b "Test Message" \
  --attach /path/to/image.png \
  "slack://xoxb-your-slack-key/#your-channel"

[caronc]

Amazing work! Thank you for this PR!