Add missing needed dependency

[jesugmz]

Installing the specified dependencies there is still a missing one under Ubuntu 18.04:

sudo  pip3 install mysqlclient
The directory '/home/jesu/.cache/pip/http' or its parent directory is not owned by the current user and the cache has been disabled. Please check the permissions and owner of that directory. If executing pip with sudo, you may want sudo's -H flag.
The directory '/home/jesu/.cache/pip' or its parent directory is not owned by the current user and caching wheels has been disabled. check the permissions and owner of that directory. If executing pip with sudo, you may want sudo's -H flag.
Collecting mysqlclient
  Downloading https://files.pythonhosted.org/packages/d0/97/7326248ac8d5049968bf4ec708a5d3d4806e412a42e74160d7f266a3e03a/mysqlclient-1.4.6.tar.gz (85kB)
    100% |████████████████████████████████| 92kB 493kB/s 
Installing collected packages: mysqlclient
  Running setup.py install for mysqlclient ... error
    Complete output from command /usr/bin/python3 -u -c "import setuptools, tokenize;__file__='/tmp/pip-build-e469qmkt/mysqlclient/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" install --record /tmp/pip-gku59by8-record/install-record.txt --single-version-externally-managed --compile:
    /usr/lib/python3.6/distutils/dist.py:261: UserWarning: Unknown distribution option: 'long_description_content_type'
      warnings.warn(msg)
    running install
    running build
    running build_py
    creating build
    creating build/lib.linux-x86_64-3.6
    creating build/lib.linux-x86_64-3.6/MySQLdb
    copying MySQLdb/__init__.py -> build/lib.linux-x86_64-3.6/MySQLdb
    copying MySQLdb/_exceptions.py -> build/lib.linux-x86_64-3.6/MySQLdb
    copying MySQLdb/compat.py -> build/lib.linux-x86_64-3.6/MySQLdb
    copying MySQLdb/connections.py -> build/lib.linux-x86_64-3.6/MySQLdb
    copying MySQLdb/converters.py -> build/lib.linux-x86_64-3.6/MySQLdb
    copying MySQLdb/cursors.py -> build/lib.linux-x86_64-3.6/MySQLdb
    copying MySQLdb/release.py -> build/lib.linux-x86_64-3.6/MySQLdb
    copying MySQLdb/times.py -> build/lib.linux-x86_64-3.6/MySQLdb
    creating build/lib.linux-x86_64-3.6/MySQLdb/constants
    copying MySQLdb/constants/__init__.py -> build/lib.linux-x86_64-3.6/MySQLdb/constants
    copying MySQLdb/constants/CLIENT.py -> build/lib.linux-x86_64-3.6/MySQLdb/constants
    copying MySQLdb/constants/CR.py -> build/lib.linux-x86_64-3.6/MySQLdb/constants
    copying MySQLdb/constants/ER.py -> build/lib.linux-x86_64-3.6/MySQLdb/constants
    copying MySQLdb/constants/FIELD_TYPE.py -> build/lib.linux-x86_64-3.6/MySQLdb/constants
    copying MySQLdb/constants/FLAG.py -> build/lib.linux-x86_64-3.6/MySQLdb/constants
    running build_ext
    building 'MySQLdb._mysql' extension
    creating build/temp.linux-x86_64-3.6
    creating build/temp.linux-x86_64-3.6/MySQLdb
    x86_64-linux-gnu-gcc -pthread -DNDEBUG -g -fwrapv -O2 -Wall -g -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -fPIC -Dversion_info=(1,4,6,'final',0) -D__version__=1.4.6 -I/usr/include/mysql -I/usr/include/python3.6m -c MySQLdb/_mysql.c -o build/temp.linux-x86_64-3.6/MySQLdb/_mysql.o
    x86_64-linux-gnu-gcc -pthread -shared -Wl,-O1 -Wl,-Bsymbolic-functions -Wl,-Bsymbolic-functions -Wl,-z,relro -Wl,-Bsymbolic-functions -Wl,-z,relro -g -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 build/temp.linux-x86_64-3.6/MySQLdb/_mysql.o -lmysqlclient -lpthread -lz -lm -lrt -latomic -lssl -lcrypto -ldl -o build/lib.linux-x86_64-3.6/MySQLdb/_mysql.cpython-36m-x86_64-linux-gnu.so
    /usr/bin/ld: cannot find -lssl
    /usr/bin/ld: cannot find -lcrypto
    collect2: error: ld returned 1 exit status
    error: command 'x86_64-linux-gnu-gcc' failed with exit status 1
    
    ----------------------------------------
Command "/usr/bin/python3 -u -c "import setuptools, tokenize;__file__='/tmp/pip-build-e469qmkt/mysqlclient/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" install --record /tmp/pip-gku59by8-record/install-record.txt --single-version-externally-managed --compile" failed with error code 1 in /tmp/pip-build-e469qmkt/mysqlclient/

Just add it as required dependencies under the requirements.

[codecov]

Codecov Report

Merging #420 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #420   +/-   ##
=======================================
  Coverage   86.75%   86.75%           
=======================================
  Files          12       12           
  Lines        1533     1533           
=======================================
  Hits         1330     1330           
  Misses        203      203

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[methane]

mysqlclient doesn't depend on libssl-dev. The error is caused by mysql_config.
It is fixed already in #409.