Try to fix buildbot failures on old OpenSSLs (< 1.0.0) - followup to issue #21015
Address issue 18229 - Explain http.server.BaseHTTPRequestHandler's .headers attribute further.

Initial patch by Caelyn McAulay.
use the called property of the run_pip mock rather than an assertion method that doesn't exist
remove superfluous and useless line
#18628: clarify index entry for source file encoding declaration.

Patch by Sam Lucidi.
Correct the URL in the http.client example. Noted by Evens Fortuné. Closes #21229
Clarify BROWSER envar behavior in webbrowser.py. Noted by David Turner. Closes #21248