version.txt is missing in /stable_baseline3

[xuewanqi]

Hi, it seems that the version.txt is missing in /stable_baseline3, which will cause a error when installing (running pip install -e .[docs,tests,extra])

[luckyhlin]

I've faced the similar issue. It looks like the .gitignore file removes *.txt from git repo.
Comment this line and add a file named verstion.txt under /stable_baseline3 with (maybe) arbitary text may be a workaround.