requirements-dev.txt: "Could not find a version that satisfies the requirement pyqt"
I followed the contributors guide and set up the dev environment without anaconda. Generally works, except for a small problem in

```
python -m pip install -r requirements-dev.txt
```
which fails with
```
Could not find a version that satisfies the requirement pyqt
```

`pyqt` is not known on PyPI and I resolved the problem by replacing it with `pyqt5`, i.e. with the following patch:

```
From a284ac89a43237784d663b2a475eead953e8a64d Mon Sep 17 00:00:00 2001
From: Jan-Philip Gehrcke <jgehrcke@googlemail.com>
Date: Wed, 12 Jun 2019 20:04:06 +0200
Subject: [PATCH] BUG: requirements-dev: Could not find ... pyqt

As of today 'pyqt' is not a valid PyPI project.

This results in the error message

    Could not find a version that satisfies the requirement pyqt

when running

    python -m pip install -r requirements-dev.txt

from the contributors guide.
---
 requirements-dev.txt | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/requirements-dev.txt b/requirements-dev.txt
index 46e857f2e9e..73b591ce8f6 100644
--- a/requirements-dev.txt
+++ b/requirements-dev.txt
@@ -44,7 +44,7 @@ html5lib
 lxml
 openpyxl
 pyarrow>=0.9.0
-pyqt
+pyqt5
 tables>=3.4.2
 python-snappy
 s3fs
@@ -52,4 +52,4 @@ sqlalchemy
 xarray
 xlrd
 xlsxwriter
-xlwt
\ No newline at end of file
+xlwt
```

But that does not pass CI because of
```
requirements-dev.txt(,): error : `requirements-dev.txt` has to be generated with `ci/../scripts/generate_pip_deps_from_conda.py` after `environment.yml` is modified.
```

From here I am "giving up" but I would at least like to leave this report.