Add dependencies in setup.py.

Add Biopython as a required dependency, and reportlab as an optional
dependency. Do not require reportlab to launch the seqtool program.

incenp/bio/seq/seqtool.py

@@ -25,7 +25,11 @@ from incenp.bio.seq import utils
 from incenp.bio.seq import wrappers
 from incenp.bio.seq.plasmidmap import summarize_vector
 from incenp.bio.seq.usa import read_usa, write_usa
-from reportlab.pdfgen.canvas import Canvas
+
+try:
+    from reportlab.pdfgen.canvas import Canvas
+except ImportError:
+    pass
 
 prog_name = "seqtool"
 prog_notice = f"""\

setup.py

@@ -38,10 +38,15 @@ setup(
         ],
 
     install_requires=[
+        'biopython',
         'click',
         'click_shell'
         ],
 
+    extras_require={
+        'plasmm': ['reportlab']
+        },
+
     packages=[
         'incenp',
         'incenp.bio',