Add an extra for mypyc dependencies (#16229)

Fixes #15579

mypyc/doc/getting_started.rst

@@ -38,17 +38,17 @@ Installation
 ------------
 
 Mypyc is shipped as part of the mypy distribution. Install mypy like
-this (you need Python 3.5 or later):
+this (you need Python 3.8 or later):
 
 .. code-block::
 
-    $ python3 -m pip install -U mypy
+    $ python3 -m pip install -U 'mypy[mypyc]'
 
 On some systems you need to use this instead:
 
 .. code-block::
 
-    $ python -m pip install -U mypy
+    $ python -m pip install -U 'mypy[mypyc]'
 
 Example program
 ---------------

setup.py

@@ -227,6 +227,7 @@ def run(self):
     # Same here.
     extras_require={
         "dmypy": "psutil >= 4.0",
+        "mypyc": "setuptools >= 50",
         "python2": "",
         "reports": "lxml",
         "install-types": "pip",