[REF] requirements.txt: Use "pymssql<=2.2.5" to fix "Failed building wheel for pymssql" (#195)

The `pymssql==2.2.6` version requires extra setup to be installed

So, it is raising new errors where it is not setup

You can reproduce it running the following command:

    docker run -it --rm  python:3.6 pip3 install -q pymssql==2.2.6

`ERROR: Could not build wheels for pymssql...`

    ERROR: Command errored out with exit status 1:
    command: /usr/local/bin/python /usr/local/lib/python3.6/site-packages/pip/_vendor/pep517/in_process/_in_process.py build_wheel /tmp/tmpmp6b33ly
        cwd: /tmp/pip-install-p4ocn1ip/pymssql_01748f191a744758bda6f7d536f6ec35
    Complete output (25 lines):
    setup.py: platform.system() => Linux
    setup.py: platform.architecture() => ('64bit', 'ELF')
    setup.py: platform.libc_ver() => ('glibc', '2.2.5')
    setup.py: include_dirs => []
    setup.py: library_dirs => []
    running bdist_wheel
    running build
    running build_py
    creating build
    creating build/lib.linux-x86_64-3.6
    creating build/lib.linux-x86_64-3.6/pymssql
    copying src/pymssql/__init__.py -> build/lib.linux-x86_64-3.6/pymssql
    running build_ext
    cythoning src/pymssql/_mssql.pyx to src/pymssql/_mssql.c
    cythoning src/pymssql/_pymssql.pyx to src/pymssql/_pymssql.c
    building 'pymssql._mssql' extension
    creating build/temp.linux-x86_64-3.6
    creating build/temp.linux-x86_64-3.6/src
    creating build/temp.linux-x86_64-3.6/src/pymssql
    gcc -pthread -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -fPIC -I/usr/local/include/python3.6m -c src/pymssql/_mssql.c -o build/temp.linux-x86_64-3.6/src/pymssql/_mssql.o -DMSDBLIB
    src/pymssql/_mssql.c:747:10: fatal error: sqlfront.h: No such file or directory
        747 | #include "sqlfront.h"
            |          ^~~~~~~~~~~~
    compilation terminated.
    error: command 'gcc' failed with exit status 1
    ----------------------------------------
    ERROR: Failed building wheel for pymssql
    ERROR: Could not build wheels for pymssql which use PEP 517 and cannot be installed directly
    WARNING: You are using pip version 21.2.4; however, version 21.3.1 is available.
    You should consider upgrading via the '/usr/local/bin/python -m pip install --upgrade pip' command.
    FAIL

However, using

    docker run -it --rm  python:3.6 pip3 install -q pymssql==2.2.5

It is installing well!

So, I have created this PR in order to fix all the error in the CI using this project and installing this package

requirements.txt

@@ -1,4 +1,4 @@
 sqlalchemy
 mysqlclient==2.0.1
-pymssql
+pymssql<=2.2.5
 sqlparse

setup/base_external_dbsource_mssql/setup.py

@@ -2,5 +2,11 @@ import setuptools
 
 setuptools.setup(
     setup_requires=['setuptools-odoo'],
-    odoo_addon=True,
+    odoo_addon={
+        'external_dependencies_override': {
+            'python': {
+                'pymssql': 'pymssql<=2.2.5',
+            }
+        },
+    }
 )