Improve development instructions

- Use a virtual environment instead of installing stuff globally (which
  doesn’t work on my system anyway).
- Remove potentially confusing prompt characters from commands.

DEVELOPMENT.md

@@ -4,11 +4,13 @@ Development
 To install in development mode with all required development and testing
 dependencies:
 
-    # pip3 install -e '.[dev]'
+    python3 -m venv venv
+    source venv/bin/activate
+    pip install -e '.[dev]'
 
 To run the tests:
 
-    # ./test
+    ./test
 
 
 Packaging
@@ -19,10 +21,8 @@ will ensure that pep517 and twine are installed.
 
 Build the wheel with pep517's build command:
 
-    # python3 -m pep517.build .
+    python3 -m pep517.build .
 
 Upload new the version to PyPI:
 
-    # python3 -m twine upload dist/*
-
-
+    python3 -m twine upload dist/*