`tests` directory is installed with `pip install`

#### Problem description

**Version:** 4.2.0



**Observed:**

```bash

$ pip install smart-open --target /tmp/smart-open

$ cd /tmp/smart-open/smart_open

$ du -hcs *

```



We get:

```

16K	azure.py

8,0K	bytebuffer.py

4,0K	compression.py

4,0K	concurrency.py

4,0K	constants.py

8,0K	doctools.py

20K	gcs.py

4,0K	hdfs.py

12K	http.py

4,0K	__init__.py

4,0K	local_file.py

160K	__pycache__

44K	s3.py

16K	smart_open_lib.py

8,0K	ssh.py

524K	tests

4,0K	transport.py

8,0K	utils.py

4,0K	version.py

8,0K	webhdfs.py

864K	total

```



==> Tests files represent 60% of the total package size.

These files are not needed at "run time".



**Expected:**

==> Tests files are not installed with `$ pip install`. (So the size of the package goes down to 340 kB.)



**Additional information**:

If this issue is validated, I can fix it and provide a PR.

The size of packages are important when using AWS Lambdas. (We want to keep Lambdas size as small as possible.)



#### Checklist



Before you create the issue, please make sure you have:



- [X] Described the problem clearly

- [X] Provided a minimal reproducible example, including any required data

- [X] Provided the version numbers of the relevant software

[]