Always download the pinned version of pip in pipstrap (#6691)

This will immediately address the breakage reported in #6682 and tracked at #6685. Virtualenv downloads the latest pip, which causes issues, so after virtualenv upgrades pip, downgrade to the pinned version.

I've confirmed that this fixes the issue on a machine that fails with the version of certbot-auto currently in master: recent version of virtualenv, python 2.7.

* Always download the pinned version of pip in pipstrap

* Run build.py

* Update changelog

* Remove unused variable

* Run build.py

CHANGELOG.md

@@ -19,6 +19,7 @@ Certbot adheres to [Semantic Versioning](https://semver.org/).
 * Fixed accessing josepy contents through acme.jose when the full acme.jose
   path is used.
 * Clarify behavior for deleting certs as part of revocation.
+* Always download the pinned version of pip in pipstrap to address breakages
 * Rename old,default.conf to old-and-default.conf to address commas in filenames
   breaking recent versions of pip.
 * Add VIRTUALENV_NO_DOWNLOAD=1 to all calls to virtualenv to address breakages

letsencrypt-auto-source/letsencrypt-auto

@@ -1346,9 +1346,6 @@ def get_index_base():
 def main():
     pip_version = StrictVersion(check_output(['pip', '--version'])
                                 .decode('utf-8').split()[1])
-    min_pip_version = StrictVersion(PIP_VERSION)
-    if pip_version >= min_pip_version:
-        return 0
     has_pip_cache = pip_version >= StrictVersion('6.0')
     index_base = get_index_base()
     temp = mkdtemp(prefix='pipstrap-')

letsencrypt-auto-source/pieces/pipstrap.py

@@ -148,9 +148,6 @@ def get_index_base():
 def main():
     pip_version = StrictVersion(check_output(['pip', '--version'])
                                 .decode('utf-8').split()[1])
-    min_pip_version = StrictVersion(PIP_VERSION)
-    if pip_version >= min_pip_version:
-        return 0
     has_pip_cache = pip_version >= StrictVersion('6.0')
     index_base = get_index_base()
     temp = mkdtemp(prefix='pipstrap-')