Add development versioning scheming (#601)

Author: Daraan

CONTRIBUTING.md

@@ -24,6 +24,11 @@ Starting with version 4.0.0, `typing_extensions` uses
 [Semantic Versioning](https://semver.org/). See the documentation
 for more detail.
 
+## Development version
+After a release the version is increased once in [pyproject.toml](/pyproject.toml) and
+appended with a `.dev` suffix, e.g. `4.0.1.dev`.
+Further subsequent updates are not planned between releases.
+
 # Type stubs
 
 A stub file for `typing_extensions` is maintained
@@ -79,7 +84,7 @@ pipx run pre-commit run -a
 # Workflow for PyPI releases
 
 - Make sure you follow the versioning policy in the documentation
-  (e.g., release candidates before any feature release)
+  (e.g., release candidates before any feature release, do not release development versions)
 
 - Ensure that GitHub Actions reports no errors.
 
@@ -93,3 +98,5 @@ pipx run pre-commit run -a
 
 - Release automation will finish the release. You'll have to manually
   approve the last step before upload.
+
+- After the release has been published on PyPI upgrade the version in number in [pyproject.toml](/pyproject.toml) to a `dev` version of the next planned release. For example, change 4.1.1 to 4.X.X.dev, see also [Development versions](#development-version). # TODO decide on major vs. minor increase.