Merge branch '5.1' into 5.2

* 5.1:
  [travis] always install ext-mongodb
  Fix circular loop with EntityManager

Author: nicolas-grekas

composer.json

@@ -51,6 +51,8 @@
         "twig/markdown-extra": "^2.12"
     },
     "conflict": {
+        "phpdocumentor/reflection-docblock": "<3.2.2",
+        "phpdocumentor/type-resolver": "<1.4.0",
         "symfony/console": "<4.4",
         "symfony/form": "<5.1",
         "symfony/http-foundation": "<4.4",