Repository: scrapy/scrapy

Issue:
Install on Python 3.4 is failing
[lxml 4.4.0](https://pypi.org/project/lxml/4.4.0/) dropped support for Python 3.4 so installing Scrapy using `pip install Scrapy` is failing.

```
(.venv) renne@girafa:~/projects/tmp/test-scrapy-py34 $ pip install scrapy
DEPRECATION: Python 3.4 support has been deprecated. pip 19.1 will be the last one supporting it. Please upgrade your Python as Python 3.4 won't be maintained after March 2019 (cf PEP 429).
Collecting scrapy
  Using cached https://files.pythonhosted.org/packages/a3/b1/d1ab5b3f84640097cf5ff642e2e357546781746d4fec2ebb40432904c57d/Scrapy-1.7.2-py2.py3-none-any.whl
Collecting pyOpenSSL (from scrapy)
  Using cached https://files.pythonhosted.org/packages/01/c8/ceb170d81bd3941cbeb9940fc6cc2ef2ca4288d0ca8929ea4db5905d904d/pyOpenSSL-19.0.0-py2.py3-none-any.whl
Collecting Twisted<=19.2.0,>=13.1.0; python_version == "3.4" (from scrapy)
  Using cached https://files.pythonhosted.org/packages/f8/2b/a80a70f71eb2b86992ffa5aaae41457791ae67faa70927fd16b76127c2b7/Twisted-19.2.0.tar.bz2
Collecting parsel>=1.5 (from scrapy)
  Using cached https://files.pythonhosted.org/packages/96/69/d1d5dba5e4fecd41ffd71345863ed36a45975812c06ba77798fc15db6a64/parsel-1.5.1-py2.py3-none-any.whl
Requirement already satisfied: six>=1.5.2 in ./.venv/lib/python3.4/site-packages (from scrapy) (1.12.0)
Requirement already satisfied: PyDispatcher>=2.0.5 in ./.venv/lib/python3.4/site-packages (from scrapy) (2.0.5)
Collecting w3lib>=1.17.0 (from scrapy)
  Using cached https://files.pythonhosted.org/packages/81/43/9dcf92a77f5f0afe4f4df2407d7289eea01368a08b64bda00dd318ca62a6/w3lib-1.20.0-py2.py3-none-any.whl
Collecting cssselect>=0.9 (from scrapy)
  Using cached https://files.pythonhosted.org/packages/7b/44/25b7283e50585f0b4156960691d951b05d061abf4a714078393e51929b30/cssselect-1.0.3-py2.py3-none-any.whl
Collecting service-identity (from scrapy)
  Using cached https://files.pythonhosted.org/packages/e9/7c/2195b890023e098f9618d43ebc337d83c8b38d414326685339eb024db2f6/service_identity-18.1.0-py2.py3-none-any.whl
Requirement already satisfied: queuelib in ./.venv/lib/python3.4/site-packages (from scrapy) (1.5.0)
Collecting lxml (from scrapy)
  Using cached https://files.pythonhosted.org/packages/e1/f5/5eb3b491958dcfdcfa5daae3c655ab59276bc216ca015e44743c9c220e9e/lxml-4.4.0.tar.gz
    ERROR: Complete output from command python setup.py egg_info:
    ERROR: This lxml version requires Python 2.7, 3.5 or later.
    ----------------------------------------
ERROR: Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-install-qdo2c8yz/lxml/
```
Using [lxml 4.3.5](https://pypi.org/project/lxml/4.3.5/) works fine.


Discussion:

Resolving patch:
diff --git a/setup.py b/setup.py
--- a/setup.py
+++ b/setup.py
@@ -69,7 +69,8 @@ def has_environment_marker_platform_impl_support():
         'Twisted>=13.1.0,<=19.2.0;python_version=="3.4"',
         'w3lib>=1.17.0',
         'queuelib',
-        'lxml',
+        'lxml;python_version!="3.4"',
+        'lxml<=4.3.5;python_version=="3.4"',
         'pyOpenSSL',
         'cssselect>=0.9',
         'six>=1.5.2',


Test patch:
diff --git a/tests/constraints.txt b/tests/constraints.txt
--- a/tests/constraints.txt
+++ b/tests/constraints.txt
@@ -1,2 +1 @@
-Twisted!=18.4.0
-lxml!=4.2.2
\ No newline at end of file
+Twisted!=18.4.0
\ No newline at end of file