pythonlib: fix pygrass Module tests by making it picklable

[petrasovaa]

To make it picklable, set popen attribute to None and add __reduce__ to TypeDict.
Slightly changes API by making popen a private variable.

[petrasovaa]

This should fix https://trac.osgeo.org/grass/ticket/3799

[wenzeslaus]

I think some related fixes are needed in t.rast.accumulate code which now fails with AttributeError: 'Module' object has no attribute 'popen', for example in this context:

Running test.t.rast.accdetect.reverse (./temporal/t.rast.accdetect)...
========================================================================
+ g.region s=0 n=80 w=0 e=120 b=0 t=50 res=10 -p
+ export GRASS_OVERWRITE=1
+ r.mapcalc expr=temp_6 = 5
+ r.mapcalc expr=temp_5 = 10
+ r.mapcalc expr=temp_4 = 15
+ r.mapcalc expr=temp_3 = 20
+ r.mapcalc expr=temp_2 = 25
+ r.mapcalc expr=temp_1 = 30
+ t.create type=strds temporaltype=absolute output=temp_abs1 title=A test descr=A test
Default TGIS driver / database set to:
driver: sqlite
database: $GISDBASE/$LOCATION_NAME/$MAPSET/tgis/sqlite.db
WARNING: Temporal database connection defined as:
/home/runner/nc_spm_full_v2alpha2/__temporal_t_rast_accdetect_test.t.rast.accdetect.reverse/tgis/sqlite.db
But database file does not exist.
Creating temporal database: /home/runner/nc_spm_full_v2alpha2/__temporal_t_rast_accdetect_test.t.rast.accdetect.reverse/tgis/sqlite.db
+ t.register -i type=raster input=temp_abs1 maps=temp_1,temp_2,temp_3,temp_4,temp_5,temp_6 start=2001-01-01 increment=2 months
Gathering map information...
0..ERROR: Unable to read band reference file for raster map <temp_1@__temporal_t_rast_accdetect_test.t.rast.accdetect.reverse>
ERROR: Unable to read band reference file for raster map <temp_2@__temporal_t_rast_accdetect_test.t.rast.accdetect.reverse>
ERROR: Unable to read band reference file for raster map <temp_3@__temporal_t_rast_accdetect_test.t.rast.accdetect.reverse>
ERROR: Unable to read band reference file for raster map <temp_4@__temporal_t_rast_accdetect_test.t.rast.accdetect.reverse>
ERROR: Unable to read band reference file for raster map <temp_5@__temporal_t_rast_accdetect_test.t.rast.accdetect.reverse>
ERROR: Unable to read band reference file for raster map <temp_6@__temporal_t_rast_accdetect_test.t.rast.accdetect.reverse>
100
Registering maps in the temporal database...
Registering maps in the space time dataset...
0..Updating space time dataset...
100
+ t.rast.accumulate -r input=temp_abs1 output=temp_accumulation base=temp_acc limits=10,25 start=2001-01-01 gran=2 months cycle=12 months suffix=num%03
Processing cycle 2001-01-01 00:00:00 - 2002-01-01 00:00:00
0..12..25..37..50..62..75..87..100
Traceback (most recent call last):
  File "scripts/t.rast.accumulate", line 569, in <module>
    main()
  File "scripts/t.rast.accumulate", line 499, in main
    if accmod.popen.returncode != 0:
AttributeError: 'Module' object has no attribute 'popen'
========================================================================
test.t.rast.accdetect.reverse from ./temporal/t.rast.accdetect failed
Running test.t.rast.accdetect (./temporal/t.rast.accdetect)...
========================================================================
+ g.region s=0 n=80 w=0 e=120 b=0 t=50 res=10 -p
+ export GRASS_OVERWRITE=1
+ r.mapcalc expr=temp_1 = 5
+ r.mapcalc expr=temp_2 = 10
+ r.mapcalc expr=temp_3 = 15
+ r.mapcalc expr=temp_4 = 20
+ r.mapcalc expr=temp_5 = 25
+ r.mapcalc expr=temp_6 = 30
+ t.create type=strds temporaltype=absolute output=temp_abs1 title=A test descr=A test
Default TGIS driver / database set to:
driver: sqlite
database: $GISDBASE/$LOCATION_NAME/$MAPSET/tgis/sqlite.db
WARNING: Temporal database connection defined as:
/home/runner/nc_spm_full_v2alpha2/__temporal_t_rast_accdetect_test.t.rast.accdetect/tgis/sqlite.db
But database file does not exist.
Creating temporal database: /home/runner/nc_spm_full_v2alpha2/__temporal_t_rast_accdetect_test.t.rast.accdetect/tgis/sqlite.db
+ t.register -i type=raster input=temp_abs1 maps=temp_1,temp_2,temp_3,temp_4,temp_5,temp_6 start=2001-01-01 increment=2 months
Gathering map information...
0..ERROR: Unable to read band reference file for raster map <temp_1@__temporal_t_rast_accdetect_test.t.rast.accdetect>
ERROR: Unable to read band reference file for raster map <temp_2@__temporal_t_rast_accdetect_test.t.rast.accdetect>
ERROR: Unable to read band reference file for raster map <temp_3@__temporal_t_rast_accdetect_test.t.rast.accdetect>
ERROR: Unable to read band reference file for raster map <temp_4@__temporal_t_rast_accdetect_test.t.rast.accdetect>
ERROR: Unable to read band reference file for raster map <temp_5@__temporal_t_rast_accdetect_test.t.rast.accdetect>
ERROR: Unable to read band reference file for raster map <temp_6@__temporal_t_rast_accdetect_test.t.rast.accdetect>
100
Registering maps in the temporal database...
Registering maps in the space time dataset...
0..Updating space time dataset...
100
+ t.rast.accumulate input=temp_abs1 output=temp_accumulation base=temp_acc limits=10,25 start=2001-01-01 gran=2 months cycle=12 months suffix=num%03
Processing cycle 2001-01-01 00:00:00 - 2002-01-01 00:00:00
0..12..25..37..50..62..75..87..100
Traceback (most recent call last):
  File "scripts/t.rast.accumulate", line 569, in <module>
    main()
  File "scripts/t.rast.accumulate", line 499, in main
    if accmod.popen.returncode != 0:
AttributeError: 'Module' object has no attribute 'popen'
========================================================================