Many new unit tests for configuration settings file (pelicanconf.py) (#3357)

[egberts]

• Now reports line number/offset of pelicanconf.py syntax errors
• Better error messages with full absolute resolved filespec due to:
  • Read-only attribute accidentially used
  • Non-existant file report
• Prevent clobbering of Python built-in system modules by user's custom filename
• Breakout test_settings.py into smaller and manageable UT files
• test_settings.py
• test_settings_deprecated.py
• test_settings_module.py
• test_settings_path.py
• Start using pytest (test_settings_path.py) instead of unittest
• Set minimum Python to 3.8 (pytest 4.0)

Pull Request Checklist

Resolves: #3357

• [XXX] Ensured tests pass and (if applicable) updated functional test output
• [XXX] Conformed to code style guidelines by running appropriate linting tools
• [XXX] Added tests for changed code
• Updated documentation for changed code

No change in documentation.

[egberts]

test_load_source_module_path_abs_syntax_error_fail() fails parallelization of pytest due to multiple concurrent accesses to the sys.modules[]. Needs a file lock to ensure serialization testing.

Withdrawing PR.