tox.ini: Measure test coverage with nosexcover

```
$ tox -e py27
...
Name                           Stmts   Miss  Cover   Missing
------------------------------------------------------------
MySQLdb                           43      7    84%   22, 36-37, 64, 67, 70, 73
MySQLdb.connections              149     33    78%   33, 53, 158, 174, 257-259, 262-265, 284-287, 291-299, 309, 311-317, 325, 334-338
MySQLdb.constants                  1      0   100%
MySQLdb.constants.CLIENT          18      0   100%
MySQLdb.constants.ER             459      0   100%
MySQLdb.constants.FIELD_TYPE      29      0   100%
MySQLdb.constants.FLAG            16      0   100%
MySQLdb.converters                58     28    52%   42-47, 53-54, 58-59, 64, 67, 77, 82, 95, 110-124, 127, 130, 133, 185-186
MySQLdb.cursors                  276     85    69%   12-16, 110-120, 161, 181, 184, 192-198, 200, 232, 235-238, 246, 250-261, 301, 309, 330, 397-407, 410-412, 429-435, 440-445, 449-453, 456, 459-462, 483-486, 491-494, 499-502
MySQLdb.release                    3      0   100%
MySQLdb.times                     87     36    59%   47-50, 55-56, 60-63, 69-70, 77, 80-82, 85-96, 101-104, 118-126
_mysql_exceptions                 15      2    87%   10-12
capabilities                     180     17    91%   37-44, 52, 55, 63, 151, 164, 175, 214-217
configdb                          14      0   100%
dbapi20                          420    121    71%   141-142, 154-155, 163-164, 174-175, 239-240, 302-328, 332-348, 384, 389, 394, 403-409, 434, 439, 444, 453-461, 479-519, 623-669, 709, 721, 725-751, 754, 778-785, 789
decimal                         2485   2485     0%   20-6198
numbers                          132    132     0%   4-391
------------------------------------------------------------
TOTAL                           4385   2946    33%
----------------------------------------------------------------------
Ran 69 tests in 5.765s

OK
...
```