Update use of Python to default to and/or use Python 3

[elextr]

Since distros are now not defining python and python 2 is dead, should be changed to python3, see also discussion on #2223

Goals:

• Change from python to python3 in the filedefs.python file (see discussion in Allow to select the Python command, and use py on Windows #2223).
• Make the main.py template use python3. Port "main.py" template to Python3 #3485
• Switch build system to use >= 3.x. Switch to Python 3 for GTK doc header generation #2903
• Use Python3 for Docutils (ex. python3-docutils on Debians).
• Update the create_py_tags.py script to use Python 3 (Refs #2615 - Convert create_py_tags.py to Python 3 #2630 and Rewrite Python standard library tags creation script for Python 3 #3039).
• Update the gen-api-gtkdoc.py script to use Python 3. Switch to Python 3 for GTK doc header generation #2903
• Update the create_php_tags.py script to use Python 3. Port create_php_tags to Python3 and generate new PHP tags file #3488
• Update (or remove, does anybody use it? I wrote it and barely used it.) the rstrip-whitespace.py script to use Python 3. Remove obsolete helper scripts #3486
• Update the update-python-identifiers.sh script to use only Python 3 (Generate Python identifiers only for Python 3 #2924).
• OSX: Update plist_filetypes.py to use Python 3. (Done in geany/geany-osx@19bcb0a)

[elextr]

First noticed by @codebrainz not me.

[codebrainz]

I propose we expand this issue to become an omnibus tracker issue that includes:

• Change from python to python3 in the filedefs.python file (see discussion in Allow to select the Python command, and use py on Windows #2223).
• Switch build system to use >= 3.x.
• Use Python3 for Docutils (ex. python3-docutils on Debians).
• Update the create_py_tags.py script to use Python 3.
• Update the gen-api-gtkdoc.py script to use Python 3.

[elextr]

Sounds like a good idea, maybe best to change the issue title and move that to a checkpoint in the list.

Didn't know Geany had such a snake pit (a lot of pythons, dad joke).

[codebrainz]

maybe best to change the issue title and move that to a checkpoint in the list.

Done, and tagged. I might have a crack at these mostly trivial changes, will ping here if I do.

[eht16]

Not even three later, we finally managed to finish all open tasks. Python 2 R.I.P.

[elextr]

Well, mainly you managed to do it so thanks.