Prevent Heroku from using Pipenv

Heroku uses an outdated Pipenv that has started to fail hash checks.

bin/pre_compile

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+
+# Make non-zero exit codes & other errors fatal:
+# http://redsymbol.net/articles/unofficial-bash-strict-mode/
+set -euo pipefail
+
+# Prevent heroku-buildpack-python from using pipenv instead of pip and requirements.txt.
+# https://github.com/heroku/heroku-buildpack-python/issues/704
+rm Pipfile Pipfile.lock

requirements.txt

@@ -0,0 +1,43 @@
+-i https://pypi.python.org/simple
+asgiref==3.2.10
+boto3==1.14.25
+botocore==1.17.25
+defusedxml==0.6.0
+diff-match-patch==20200713
+dj-database-url==0.5.0
+django-filter==2.3.0
+django-import-export==2.3.0
+django-storages==1.9.1
+django-widget-tweaks==1.4.8
+django==3.0.8
+djangorestframework==3.11.0
+docutils==0.15.2
+easy-thumbnails==2.7
+et-xmlfile==1.0.1
+faker==4.1.1
+gunicorn==20.0.4
+jdcal==1.4.1
+jmespath==0.10.0
+markuppy==1.14
+numpy==1.19.1
+odfpy==1.4.1
+openpyxl==3.0.4
+pandas==1.0.5
+pillow==7.2.0
+plotly==4.9.0
+psycopg2-binary==2.8.5
+python-dateutil==2.8.1
+python-dotenv==0.14.0
+pytz==2020.1
+pyyaml==5.3.1
+retrying==1.3.3
+s3transfer==0.3.3
+six==1.15.0
+sqlparse==0.3.1
+tablib[html,ods,xls,xlsx,yaml]==2.0.0
+text-unidecode==1.3
+uritemplate==3.0.1
+urllib3==1.25.9 ; python_version != '3.4'
+whitenoise==5.1.0
+xlrd==1.2.0
+xlwt==1.3.0