Prefer `towncrier.toml` over `pyproject.toml` if present

[jaraco]

Fixes #81

• Extract helper function for finding the config file.
• Refactor _find_config_file for simplicity.
• Refactor to generalize the search across a list of two candidates.
• Re-order search path for config files to match towncrier.

[codecov[bot]]

Codecov Report

Merging #86 (6225620) into master (dee43d1) will increase coverage by 5.66%. Report is 5 commits behind head on master. The diff coverage is 59.57%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #86 +/- ## ========================================== + Coverage 50.00% 55.66% +5.66% ========================================== Files 9 11 +2 Lines 194 212 +18 Branches 26 26 ========================================== + Hits 97 118 +21 + Misses 96 93 -3 Partials 1 1 ```

[jaraco]

I welcome others to fix the lint errors, because I have no opinion on how to address them.