improve tests for version parser (#696)

The high level goal here is to improve the tests for the version parser.
Namely, we now check not just that version strings parse successfully,
but that they parse to the expected result.

We also do a few other cleanups. Most notably, `Version` is now an
opaque type so that we can more easily change its representation going
forward.

Reviewing commit-by-commit is suggested. :-)

crates/puffin-cli/src/python_version.rs

@@ -18,18 +18,18 @@ impl FromStr for PythonVersion {
         if version.is_local() {
             return Err(format!("Python version {s} is a local version"));
         }
-        if version.epoch != 0 {
+        if version.epoch() != 0 {
             return Err(format!("Python version {s} has a non-zero epoch"));
         }
-        if version.version < Version::from_release(vec![3, 7]) {
+        if version.version < Version::new([3, 7]) {
             return Err(format!("Python version {s} must be >= 3.7"));
         }
-        if version.version >= Version::from_release(vec![4, 0]) {
+        if version.version >= Version::new([4, 0]) {
             return Err(format!("Python version {s} must be < 4.0"));
         }
 
         // If the version lacks a patch, assume the most recent known patch for that minor version.
-        match version.release.as_slice() {
+        match version.release() {
             [3, 7] => {
                 debug!("Assuming Python 3.7.17");
                 Ok(Self(StringVersion::from_str("3.7.17")?))