setup to run in a repl

[airportyh]

Why?

1. Setup a replit-1.5 branch to follow upstream's 1.5 branch.
2. We want to be able to develop poetry in a repl

Using poetry to develop itself in a repl has been challenging. I decided to back out and use pip for now. Later when
we've upgrade the poetry within the Nix module we can try again.

We plan to make the default branch replit-1.5. Upstream maintains long-lived branches for each version and we'll track 1.5 going forward.

Changes

1. added .replit
2. add a fallback in __version__.py so it can work without being properly installed as a package
3. added requirements.txt file (generated via poetry export --with dev --with test -o requirements.txt, plus small tweak) to allow pip to install dependencies
4. skipped 4 tests that can't run in the current setup

Test

1. Create a blank repl via "Import from Github" using URL: https://github.com/replit/poetry.git
2. Give the repl some boost (4x or 8x)
3. git clean -f
4. Switch to branch th-repl-setup
5. pip install -r requirements.txt
6. Try running the tests: pytest
7. Try running poetry: python -m poetry