Disallow Hard-coded __version__ and Require project.version (#61)

* Disallow `__version__` Detection and Require `project.version`

* pt 2

* refactor with `PyProjectTomlBuilder`

* wording

* wording - 2

* tests: update version logic

* tests: update version logic - 2

* fix test fixture

* add tests

* misc fix

* update README

* require introspection

* misc

* importlib_metadata - 2

README.md

@@ -33,9 +33,8 @@ In order to use the action, a few files need to have the following:
 
         setup()
         ```
-    - **Note:** a `pyproject.toml` file is not needed since it will be autogenerated. However, if one does already
-      exist (
-      ie. the most common scenario), then sections and/or attributes will be inserted. Any conflicting
+    - a `pyproject.toml` file with a `[project].version` attribute (hardcoding `__version__` strings in `__init__.py`
+      files is disallowed). Other sections and attributes will be auto-inserted. Any conflicting
       sections/attributes will be overwritten.
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