🚑 Use pip<19 in CI to mitigate a build failure

Prior to this change, CIs were failing during build step as well
as any other attempts to install aiohttp from sdist (tarball).

This change creates a temporary workaround for #3581 and should be
reverted once the proper solution is found and implemented.

.appveyor.yml

@@ -13,7 +13,7 @@ install:
   # Ensure the Git Submoduldes have been pulled down too
   - git submodule update --init --recursive
 
-  - "tools/build.cmd %PYTHON%\\python.exe -m pip install -U pip wheel setuptools"
+  - "tools/build.cmd %PYTHON%\\python.exe -m pip install -U 'pip<19' wheel setuptools"
   - "tools/build.cmd %PYTHON%\\python.exe -m pip install -r requirements/cython.txt"
   - "tools/build.cmd %PYTHON%\\python.exe -m pip install -r requirements/ci.txt"
 

.travis.yml

@@ -16,7 +16,7 @@ python:
 - &pypy3 pypy3.5
 
 install:
-- &upgrade_python_toolset pip install --upgrade pip wheel setuptools
+- &upgrade_python_toolset pip install --upgrade 'pip<19' wheel setuptools
 - pip install -r requirements/cython.txt
 - pip install -r requirements/ci.txt
 

Makefile

@@ -110,7 +110,7 @@ doc-spelling:
 	@make -C docs spelling SPHINXOPTS="-W -E"
 
 install:
-	@pip install -U pip
+	@pip install -U 'pip<19'
 	@pip install -Ur requirements/dev.txt
 
 .PHONY: all build flake test vtest cov clean doc