python: Consolidate requirements.txt files in a single place

We had them split according the separate use-cases:

 - testing
 - doc-gen
 - wire-gen

But that was causing new contributors to miss some dependencies when they
first got hacking. So this consolidates all of our own dependencies in a root
requirements.txt, with the notable exception of `pyln-client`, `pyln-testing`
and `pyln-proto` which are distributed as PyPI modules and therefore have
their own dependencies that need to be tracked in the module root.

Closes #3518

.travis/build.sh

@@ -36,8 +36,6 @@ pip3 install --user -U --quiet --progress-bar off \
 
 pip3 install --user -U --quiet --progress-bar off \
      -r requirements.txt \
-     -r tests/requirements.txt \
-     -r doc/requirements.txt \
      -r contrib/pyln-client/requirements.txt \
      -r contrib/pyln-proto/requirements.txt \
      -r contrib/pyln-testing/requirements.txt

Makefile

@@ -268,7 +268,7 @@ check: check-units installcheck pytest
 
 pytest: $(ALL_PROGRAMS)
 ifeq ($(PYTEST),)
-	@echo "py.test is required to run the integration tests, please install using 'pip3 install -r tests/requirements.txt', and rerun 'configure'."
+	@echo "py.test is required to run the integration tests, please install using 'pip3 install -r requirements.txt', and rerun 'configure'."
 	exit 1
 else
 # Explicitly hand DEVELOPER and VALGRIND so you can override on make cmd line.

doc/HACKING.md

@@ -165,9 +165,7 @@ pip3 install --user \
          -r requirements.txt \
          -r contrib/pyln-client/requirements.txt \
          -r contrib/pyln-proto/requirements.txt \
-         -r contrib/pyln-testing/requirements.txt \
-         -r tests/requirements.txt \
-         -r doc/requirements.txt
+         -r contrib/pyln-testing/requirements.txt
 ```
 
 Re-run `configure` for the python dependencies

doc/INSTALL.md

@@ -55,7 +55,7 @@ Clone lightning:
 For development or running tests, get additional dependencies:
 
     sudo apt-get install -y valgrind python3-pip libpq-dev
-    sudo pip3 install -r tests/requirements.txt -r doc/requirements.txt
+    sudo pip3 install -r requirements.txt
 
 Build lightning:
 

requirements.txt

@@ -1 +1,21 @@
+Flask==1.1.1
+Sphinx==1.8.4
+cheroot==8.2.1
+commonmark==0.8.1
+ephemeral-port-reserve==1.1.1
+flake8==3.7.8
+flaky==3.6.1
+m2r==0.2.1
 mako==1.0.14
+mrkd==0.1.6
+psycopg2-binary==2.8.4
+pytest-benchmark==3.2.2
+pytest-forked==1.0.2
+pytest-timeout==1.3.3
+pytest-xdist==1.29.0
+pytest==5.3.1
+python-bitcoinlib==0.10.2
+recommonmark==0.5.0
+sphinx-rtd-theme==0.4.2
+sphinxcontrib-websupport==1.1.0
+tqdm==4.32.2