[MRG+1] Update project.py removed one 'hack', seems irrelevant. (#3910)

* Update project.py removed one 'hack', seems irrelevant.

As mentioned by @Gallaecio in issue #3871, the 'hack'  is cleared. I also  double checked whether the environment variable "SCRAPY_PICKLED_SETTINGS_TO_OVERRIDE" was ever set in our codebase and it turns out we didn't set it or used it anywhere else.So I guess the 'hack' was not used in the current version. Also the name of this environment variable rather doesn't suggest it was  a boolean(it is used in  an 'if' condition which has perplexed me )

* Update project.py

* Update project.py

How about this?

* Update project.py

* Update project.py

* Update scrapy/utils/project.py

Co-Authored-By: Adrián Chaves <adrian@chaves.io>

* Update scrapy/utils/project.py

Co-Authored-By: Adrián Chaves <adrian@chaves.io>

* Update project.py

scrapy/utils/project.py

@@ -8,6 +8,7 @@ from os.path import join, dirname, abspath, isabs, exists
 from scrapy.utils.conf import closest_scrapy_cfg, get_config, init_env
 from scrapy.settings import Settings
 from scrapy.exceptions import NotConfigured
+from scrapy.exceptions import ScrapyDeprecationWarning
 
 ENVVAR = 'SCRAPY_SETTINGS_MODULE'
 DATADIR_CFG_SECTION = 'datadir'
@@ -70,6 +71,9 @@ def get_project_settings():
     # XXX: remove this hack
     pickled_settings = os.environ.get("SCRAPY_PICKLED_SETTINGS_TO_OVERRIDE")
     if pickled_settings:
+        warnings.warn("Use of environment variable "
+                      "'SCRAPY_PICKLED_SETTINGS_TO_OVERRIDE' "
+                      "is deprecated.", ScrapyDeprecationWarning)
         settings.setdict(pickle.loads(pickled_settings), priority='project')
 
     # XXX: deprecate and remove this functionality