Copy requirements files to .in

We’re going to move to using pip-tools for freezing requirements.

pip-tools uses `.in` files for the un-frozen list of requirements, and
then generates `.txt` equivalents.

This commit just copies our existing `.txt` files, keeping the same name
but giving them a `.in` extension ready for pip-tools to use.

requirements.in

@@ -0,0 +1,32 @@
+# Run `make freeze-requirements` to update requirements.txt
+# with package version changes made in requirements-app.txt
+
+ago==0.0.93
+govuk-bank-holidays==0.8
+humanize==3.2.0
+Flask==1.1.2
+Flask-WTF==0.14.3
+Flask-Login==0.5.0
+
+blinker==1.4
+pyexcel==0.6.6
+pyexcel-io==0.6.4
+pyexcel-xls==0.6.2
+pyexcel-xlsx==0.6.0
+pyexcel-ods3==0.6.0
+pytz==2020.5
+gunicorn==20.0.4
+eventlet==0.30.0
+notifications-python-client==5.7.1
+Shapely==1.7.1
+
+# PaaS
+awscli-cwlogs>=1.4,<1.5
+itsdangerous==1.1.0
+
+git+https://github.com/alphagov/notifications-utils.git@43.7.0#egg=notifications-utils==43.7.0
+git+https://github.com/alphagov/govuk-frontend-jinja.git@v0.5.1-alpha#egg=govuk-frontend-jinja==0.5.1-alpha
+
+# gds-metrics requires prometheseus 0.2.0, override that requirement as later versions bring significant performance gains
+prometheus-client==0.9.0
+gds-metrics==0.2.4