Add pip-requirements package to python layer

[Pip requirements mode](https://github.com/Wilfred/pip-requirements.el)
for editing pip requirement files and autocompletion of PyPI Packages by
default.

contrib/lang/python/config.el

@@ -13,6 +13,7 @@
 ;; variables
 
 (spacemacs|defvar-company-backends python-mode)
+(spacemacs|defvar-company-backends pip-requirements-mode)
 
 (defvar python-enable-yapf-format-on-save nil
   "If non-nil, automatically format code with YAPF on save.")

contrib/lang/python/packages.el

@@ -21,6 +21,7 @@
     flycheck
     helm-pydoc
     hy-mode
+    pip-requirements
     pony-mode
     pyenv-mode
     pyvenv
@@ -58,6 +59,14 @@
   (defadvice anaconda-mode-goto (before python/anaconda-mode-goto activate)
     (evil-jumper--push)))
 
+(defun python/init-pip-requirements ()
+  (use-package pip-requirements
+    :init
+    (when (configuration-layer/layer-usedp 'auto-completion)
+      (push '(company-capf :with company-yasnippet)
+            company-backends-pip-requirements-mode)
+      (spacemacs|add-company-hook pip-requirements-mode))))
+
 (defun python/init-pony-mode ()
   (use-package pony-mode
     :defer t