Filter out incompatible dists (#398)

Filter out source dists and wheels whose `requires-python` from the
simple api is incompatible with the current python version.

This change showed an important problem: When we use a fake python
version for resolving, building source distributions breaks down because
we can only build with versions we actually have.

This change became surprisingly big. The tests now require python 3.7 to
be installed, but changing that would mean an even bigger change.

Fixes #388

.gitignore

@@ -13,3 +13,6 @@ target/
 
 # Use e.g. `--cache-dir cache-docker` to keep a cache across container invocations
 cache-*
+
+# python tmp files
+__pycache__