Document common fix of Poetry problems by removing egg-info (#13785)

`matrix_synapse.egg-info/`

Mentioned at https://matrix.to/#/!vcyiEtMVHIhWXcJAfl:sw1v.org/$aKy_IjrKwb70aTVZWeW_6zt0k7OIZ1YkyZpkP9uiRaM?via=matrix.org&via=element.io&via=beeper.com and many other places.

changelog.d/13785.doc

@@ -0,0 +1 @@
+Add docs for common fix of deleting the `matrix_synapse.egg-info/` directory for fixing Python dependency problems.

docs/development/dependencies.md

@@ -126,6 +126,23 @@ context of poetry's venv, without having to run `poetry shell` beforehand.
 poetry install --extras all --remove-untracked
 ```
 
+## ...delete everything and start over from scratch?
+
+```shell
+# Stop the current virtualenv if active
+$ deactivate
+
+# Remove all of the files from the current environment.
+# Don't worry, even though it says "all", this will only
+# remove the Poetry virtualenvs for the current project.
+$ poetry env remove --all
+
+# Reactivate Poetry shell to create the virtualenv again
+$ poetry shell
+# Install everything again
+$ poetry install --extras all
+```
+
 ## ...run a command in the `poetry` virtualenv?
 
 Use `poetry run cmd args` when you need the python virtualenv context.
@@ -256,6 +273,16 @@ from PyPI. (This is what makes poetry seem slow when doing the first
 `poetry install`.) Try `poetry cache list` and `poetry cache clear --all
 <name of cache>` to see if that fixes things.
 
+## Remove outdated egg-info
+
+Delete the `matrix_synapse.egg-info/` directory from the root of your Synapse
+install.
+
+This stores some cached information about dependencies and often conflicts with
+letting Poetry do the right thing.
+
+
+
 ## Try `--verbose` or `--dry-run` arguments.
 
 Sometimes useful to see what poetry's internal logic is.