Jupytext depends on markdownitpy

[mwouts]

This is an alternative to #599 which has the dependency on markdown-it-py only when Python >= 3.6.

[codecov]

Codecov Report

Merging #601 into master will decrease coverage by 0.03%.
The diff coverage is 91.48%.

@@            Coverage Diff             @@
##           master     #601      +/-   ##
==========================================
- Coverage   99.00%   98.97%   -0.04%     
==========================================
  Files          90       90              
  Lines        8980     9000      +20     
==========================================
+ Hits         8891     8908      +17     
- Misses         89       92       +3     

Impacted Files	Coverage Δ
jupytext/myst.py	96.71% <91.48%> (-0.70%)	⬇️
jupytext/reraise.py	25.00% <0.00%> (-50.00%)	⬇️
jupytext/formats.py	96.91% <0.00%> (+0.30%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update bb697ef...883428d. Read the comment docs.

[chrisjsewell]

Yep that looks fine thanks. Two minor things:

As you see in the CI, if you make a PR to master from an "internal" branch you get duplication of runs. To remove this we use:

on:
  push:
    branches: [master]
    tags:
      - '*'
  pull_request:

I made #602, to also update the import error message and documentation

[chrisjsewell]

Another FYI:
I have recently found using tox to be really helpful for not having to manually create/maintain development environment (given that I'm working on over 10 different repos!) and have added them to most projects

Here's the tox.ini I'm using for jupytext:

# To use tox, see https://tox.readthedocs.io
# Simply pip or conda install tox
# If you use conda, you may also want to install tox-conda
# then run `tox` or `tox -- {pytest args}`
# To run in parallel using `tox -p` (this does not appear to work for this repo)

# To rebuild the tox environment, for example when dependencies change, use
# `tox -r`

# Note: if the following error is encountered: `ImportError while loading conftest`
# then then deleting compiled files has been found to fix it: `find . -name \*.pyc -delete`

[tox]
envlist = py{35,36,37,38}

[testenv:py{35,36,37,38}]
; recreate = false
deps = -rrequirements-dev.txt
commands = pytest {posargs}

[testenv:docs-{update,clean}]
deps = -rdocs/doc-requirements.txt
whitelist_externals = rm
commands =
    clean: rm -rf docs/_build
    sphinx-build {posargs} -nW --keep-going -b html docs/ docs/_build/html

[mwouts]

Thank you @chrisjsewell ! This is very helpful. I'll integrate this PR, as well as your update on the documentation and warnings, and open a follow-up issue to keep track of your multiple suggestions 😄 .

[chrisjsewell]

Thanks @mwouts 😄
No rush, but when can we expect a v1.6?

[mwouts]

Well if everything goes well, it could come within one day or two... fingers crossed!

[mwouts]

A release candidate is available here:

pip install jupytext==1.6.0rc0

[chrisjsewell]

awesome thanks 😄