tweak electrum-env script

- set -e, and don't call deactivate (not needed; and with -e
  if ./run_electrum errors it wouldn't run anyway)
- re PYTHONPATH
    - I think the sane thing is to give priority to the virtualenv,
      and only use system-packages as a fallback
    - added more paths; tested that it now works for modern Ubuntu
      and Manjaro
- use "python3 -m venv" instead of "virtualenv"
  (as former is always(?) available now)

electrum-env

@@ -4,24 +4,36 @@
 # python dependencies before activating the env and running Electrum.
 # If 'env' already exists, it is activated and Electrum is started
 # without any installations. Additionally, the PYTHONPATH environment
-# variable is set properly before running Electrum.
+# variable is set so that system packages such as e.g. apt installed
+# PyQt5 will also be visible.
 #
-# python-qt and its dependencies will still need to be installed with
-# your package manager.
+# By default, only pure python dependencies are installed.
+# If you would like more extras to be installed, do e.g.:
+# $ source ./env/bin/activate
+# $ pip install -e '.[crypto,gui,hardware]'
+# $ deactivate
+
+set -e
 
 PYTHON_VER="$(python3 -c 'import sys; print(sys.version[:3])')"
 
 cd $(dirname $0)
 if [ -e ./env/bin/activate ]; then
     source ./env/bin/activate
+    # FIXME what if this is an old directory and our requirements
+    #       changed in the meantime? should run "pip install -e . --upgrade"
 else
-    virtualenv env -p `which python3`
+    python3 -m venv env
     source ./env/bin/activate
-    python3 -m pip install .[fast]
+    pip install -e .
 fi
 
-export PYTHONPATH="/usr/local/lib/python${PYTHON_VER}/site-packages:$PYTHONPATH"
+export PYTHONPATH="$PYTHONPATH:"\
+"/usr/local/lib/python${PYTHON_VER}/site-packages:"\
+"/usr/local/lib/python${PYTHON_VER}/dist-packages:"\
+"/usr/lib/python3/dist-packages:"\
+"/usr/lib/python${PYTHON_VER}/site-packages:"\
+"${HOME}/.local/lib/python${PYTHON_VER}/site-packages"
 
-./run_electrum "$@"
 
-deactivate
+./run_electrum "$@"