Document that "coding: utf-8" is supported in requirements.txt

Fixes #7182

docs/html/reference/pip_install.rst

@@ -148,6 +148,9 @@ and the newline following it is effectively ignored.
 
 Comments are stripped *before* line continuations are processed.
 
+To interpret the requirements file in UTF-8 format add a comment
+``# -*- coding: utf-8 -*-`` to the first or second line of the file.
+
 The following options are supported:
 
   *  :ref:`-i, --index-url <--index-url>`

news/7182.doc

@@ -0,0 +1 @@
+Document that "coding: utf-8" is supported in requirements.txt

src/pip/_internal/req/req_file.py

@@ -509,6 +509,7 @@ def get_file_content(url, comes_from=None, session=None):
     # type: (str, Optional[str], Optional[PipSession]) -> Tuple[str, Text]
     """Gets the content of a file; it may be a filename, file: URL, or
     http: URL.  Returns (location, content).  Content is unicode.
+    Respects # -*- coding: declarations on the retrieved files.
 
     :param url:         File path or url.
     :param comes_from:  Origin description of requirements.