Write a replacement for pipstrap (#8401)

* Add a new, simplified version of pipstrap.

* Use tools/pipstrap.py

* Uncomment code

* Refactor pip_install.py and provide hashes.

* Fix test_sdists.sh.

* Make code work on Python 2.

* Call strip_hashes.py using Python 3.

* Pin the oldest version of httplib2 used in distros

* Strip enum34 dependency.

* Remove pip pinnings from dev_constraints.txt

* Correct pipstrap docstring.

* Don't set working_dir twice.

* Add comments

tox.ini

@@ -62,7 +62,7 @@ source_paths =
 [testenv]
 passenv =
     CERTBOT_NO_PIN
-commands_pre = python {toxinidir}/letsencrypt-auto-source/pieces/pipstrap.py
+commands_pre = python {toxinidir}/tools/pipstrap.py
 commands =
     !cover: {[base]install_and_test} {[base]all_packages}
     !cover: python tests/lock_test.py