fix(pypi): use better version detection

Closes #4047

lib/datasource/pypi/index.js

@@ -116,7 +116,7 @@ async function getSimpleDependency(depName, hostUrl) {
     const links = root.querySelectorAll('a');
     const versions = new Set();
     for (const link of links) {
-      const result = extractVersionFromLinkText(link.text);
+      const result = extractVersionFromLinkText(link.text, depName);
       if (result) {
         versions.add(result);
       }
@@ -136,11 +136,11 @@ async function getSimpleDependency(depName, hostUrl) {
   }
 }
 
-function extractVersionFromLinkText(text) {
-  const versionRegexp = /\d+(\.\d+)+/;
-  const result = text.match(versionRegexp);
-  if (result && result.length > 0) {
-    return result[0];
+function extractVersionFromLinkText(text, depName) {
+  const prefix = `${depName}-`;
+  const suffix = '.tar.gz';
+  if (!(text.startsWith(prefix) && text.endsWith(suffix))) {
+    return null;
   }
-  return null;
+  return text.replace(prefix, '').replace(/\.tar\.gz$/, '');
 }