Implements issue #268. Incorporates PR #267 and PR #265

Summary of changes:

• Unpin Pytest version.
• Add Python slim Docker containers for tags 3.10.13, 3.11.7, 3.12.1, and 3.13.0a2 to the test matrix.
  As for PR Fix PyShp's CI, dropping actions/setup-python, running build.yml Action in Python Docker containers.  #267:
• Drop actions/setup-python, move the matrix strategy forward, pin to the specific patch versions below, and use the matrix to run the action in Python Slim containers (versions 2.7.18, 3.5.10, 3.6.15, 3.7.17, 3.8.18, and 3.9.18)
• Bump actions/checkout from v2 to v3
• Don't track my VS Code settings files.
  As for PR Reformat code block as markdown literal code, so doctest skips it. Fixes #264 #265
• Reformat PyShp usage example with undefined variable in readme.md from >>> to Markdown code literal block, so doctest skips it, as for PR # 265.