templates/package: fix template to use triple quotes

[BurntSushi]

I noticed that when using a requirement like

c<2.0.0 ; sys_platform == 'linux'

then the single quotes seem to get rewritten as double quotes. And since
the TOML template just prints the value as-is without any escaping, it
ends up producing an invalid TOML file.

I do wonder whether a better fix here would be to abdicate templates and
use a proper TOML serializer from an in-memory data structure, but this
fix is probably good enough for now. Namely, we use triply quoted TOML
strings. The only way this can go wrong is if the requirement contains
three single quotes in succession.

[BurntSushi]

It took me a fair bit of time to figure out that poetry run pytest --snapshot-update was what I needed to run to update the snapshots. Is there a place in the docs where it would make sense to document this command?

(This might have just been ecosystem ignorance on my part. What actually took me a bit to figure out was that syrupy was what was doing the snapshotting. This was surprisingly difficult to difficult to discover because import syrupy never occurs anywhere.)

[BurntSushi]

It also seems to have updated a snapshot on my local system in a way that differs from CI. In particular, I had to back this snapshot update out (test_index.ambr):

@@ -1,12 +1,11 @@
 # serializer version: 1
 # name: test_index_down
   dict({
-    'exit_code': 0,
+    'exit_code': 1,
     'stderr': '''
-      Stopping server with pid [PID]...
-      Stopped server!
+      Server looks shutdown already.
   
     ''',
     'stdout': '',
   })
 # ---

[BurntSushi]

I've added a very lightweight CONTRIBUTING.md file to document some of my learnings here.